There is a lot of high quality, free material out there. The books, videos, sounds, art, and much more, at links below represent a small fraction of what's available. Some are like a fire-hydrant; others are neatly curated. Take some time to explore & find something that suits your needs.

To be included in the list, material had to be in the public domain, Open Access, or have a Creative Commons license. Be sure to double-check the specific license if you want to use the material commercially.

There are a few sites that deserve special consideration.

1. The Internet Archive, perhaps the best online non-profit library we have available to us. Films, television, music, art, books, and more – enough to last you several lifetimes.
2. Wikimedia Commons, which has more than 55,000,000 freely usable media files.
3. Europeana, which has contributions from more than 3,000 European institutions.
4. Public Domain Review, which does a marvelous job at curating high quality content.

Art

• 1 Million Free Pictures: A picture is worth a thousand words, a domain is worth a million pictures.
• 4 Free Photos: More than 300 pages of free photos that can be used personally or commercially.
• Agricultural Research Service: More than 6,500 images. They are in the public domain and copyright free unless otherwise noted.
• Avopix: More than 200,000 free images.
• Bildgeist: In their own words, Bildgeist is a “visual journal of scientific illustrations, illuminated manuscripts, photographs, and artworks from the public domain”. All art there is thought to be in the public domain. They focus on works of zoology, botany, astronomy, medicine, anatomy, cartography, alchemy, mysticism, the occult, ethnology, mythology, and art history.
• Burst: Free commercial photos made available by Shopify.
• Duion: Free textures, photos, and more.
• Foodies Feed: Over 900 tasty, CC0 licensed images.
• Clipart Spy: Public domain clipart. They suggest that you do some copyright research before using images commercially.
• Free Images: Like the name implies! Millions of public domain images, art, and vector images.
• FreeQration: More than 2,500,000 free photos, vectors, and illustrations.
• Free Vintage Illustrations - Thousands of vintage illustrations.
• Getty: Getty Open Content Is a collection of over 100,000 works that are available in the public domain. There are other images, but they come with their own restrictions.
• Gratisography: Gratisography offers free hi-res images for personal and commercial projects. All photos are CC0 licensed.
• ISO Republic: ISO Republic provides thousands of CC0 stock photos & videos. Photos are broken down into easily understandable categories to help your search.
• Life Of Pix: Free, high resolution pictures.
• Morguefile: More than 350,000 free stock photos available for commercial use.
• National Gallery of Art: The repository for the National Gallery of Art. Has more than 51,000 open access images.
• Negative Space: Negative Space offers easily search-able categories, totaling 77 (as of this writing) pages worth of photos. All photos are CC0 licensed.
• New Old Stock: Vintage photos, free of known copyright restrictions.
• New York Public Library: More than 180,000 of the items in the digital collection of the New York Public Library are in the public domain. They have a search-able database as well as an API for developers.
• National Oceanic and Atmospheric Administration: Repository for NOAA photos. Has more than 80,000 pictures.
• Old Book Illustrations: Quainter than the other sites, Old Book Illustrations has a focus on Victorian and French Romantic illustrations. There are 274 pages of wonderful illustrations, each with a bit of information along the left side of the page. The ‘Advanced Search’ page will help you narrow down what you’re looking for.
• Open Access by the Metropolitan Museum of Art: Open Access by The Met in New York has more than 400,000 works of public domain art available. Be sure to check “Public Domain Artworks” along the left side to ensure you’re browsing the proper collection. Other art may be open, but have other restrictions (attribution, non-commercial, etc.)
• PDPics: Thousands of free, public domain images.
• Pexels: Free stock photos & videos.
• Pickup Image: Hi-res public domain photos.
• Picjumbo: High resolution and free stock photos.
• Pixabay: Pixaby is home to over 1.4 million royalty free stock photos and videos. All content there is released under a Creative Commons CC0 license – meaning it’s in the public domain.
• Public Domain Archive: Categorized like many other sites, all photos here are CC0 licensed.
• Public Domain Files: More than 200,000 public domain files.
• Public Domain Photos: More than 5,000 free photos and 8,000 free cliparts.
• Public Domain Vectors: Public Domain Vectors provides 50,000 vector images, all of which are in the public domain. They come in .eps, .svg, .ai, and .cdr formats.
• RawPixel: Thousands of digitally enhanced photographic representations of existing material.
• Realistic Shots: Realistic Shots uploads seven pictures every week. All are released under a CC0 license.
• ScatterJar: Free food photos.
• SnappyGoat: Almost 14,000,000 public domain images, photos, and clipart.
• SnapwireSnaps: 7 photos every 7 days.
• SpaceX: Space X puts its own images into the public domain. They are available in a media gallery on the SpaceX website or through their Flickr page.
• Splitshire: More than 1,000 gorgeous, free photos.
• Startup Stock Photos: Like they say in their About page, “Free photos for startups, bloggers, publishers, websites, designers, developers, creators, & everyone else.” All photos are licensed under a CC0 license.
• Stocksnap: Stocksnap offers easily downloadable, high quality pictures. They’re broken down into several easy navigable categories. All photos are CC0 licensed.
• The British Library: The British Library generously put over 1,000,000 images into the public domain. A wide variety of these are images from works in the 17th, 18th, and 19th centuries – works on which the copyright has naturally expired, but are rare or otherwise unobtainable books. The British Library believes all these works to be in the public domain in most territories, but better to check and be safe.
• The Pic Pac: While not a large collection like the others, The Pic Pac has some beautiful, hi-res photos. All photos are CC0 licensed.
• United States Antarctic Program: More than 7,700 photos with an Antarctic focus.
• United States National Library of Medicine: Photos from the National Library of Medicine.
• Unsplash: Unsplash is a popular site for hi-res public domain pictures, generously gifted to the public domain by their respective photographers. While credit is appreciated, it’s not required. They also offer an API for developers.
• Viintage: Hundreds of thousands of copyright free vintage graphics.

Audio

• ccMixter: A popular site for finding Creative Commons licensed music for any kind of project you're working on.
• ChoralWiki: ChoralWiki is the Choral Public Domain Library, a collection of almost 30,000 choral works, and close to 20,000 texts. All of the material is, as implied by the proper name, in the public domain.
• FreePD: Provides CC0 1.0 Licensed Public Domain music. Everything is available for free but if you pay, you can downloads hundreds of tracks at a single time.
• FreeSound: FreeSound offers thousands of audio files, all for free. They typically fall one of several Creative Commons licensed. CC0 licensed content is in the public domain.
• Historic Music - Historic music that is thought to be out of copyright, provided by the University of Houston
• Jazz Online: More than 40,000 downloadable classic jazz songs.
• LibriVox: LibriVox is like Project Gutenberg for audiobook. All audiobooks here are in the public domain.
• Musopen: Musopen provides recordings, sheet music, and books to the public for free. All material is in the public domain. This is an especially fantastic resource for those learning how to play music.
• Mutopia Project: A fantastic, almost 2,100 strong collection of public domain or other Creative Commons-licensed sheet music. Available in .pdf, MIDI, and LilyPond formats.
• Open Music Archive: A collaborative project that sources, digitizes, and distributes out of copyright recordings. Content is either public domain or CC BY-SA 4.0.
• Opsound - Provides Creative Commons licensed music (some public domain, some not; make sure to check).
• Public Domain Information Project: A collection of public domain music and royalty free music.
• The International Music Score Library Project: An extensive (and rather inexhaustible!) collection of scores, records, and other works. All material is Creative Commons Attribution-ShareAlike 4.0 licensed. This means that the work is available for free and useable however you’d like, but you must give appropriate credit and distribute your own contributions under the same license.

Books

• Authorama: Authorama has a list of public domain and Creative Commons licensed books. Pay attention to the particular license used; but besides that, the books are available for free.
• Bartleby: Bartleby offers a wide selection of books such as The Harvard Classics, all for free.
• Classic Literature Library: The Classic Literature Library several broad categories where you can go hunting for free books.
• Digital Public Library of America - You can check the rights of a work on its page, but there’s a plethora of public domain content, waiting to be accessed.
• Hathitrust - A partnership between many universities and libraries, HathiTrust is a repository of more than 13.7 million volumes, 5.3 million of which are in the public domain.
• Google Books: A large collection of free, public domain books are available with more added yearly.
• Online Books Page - The Online Books Page lists over 2 million books, available for free. The creator of the websites copyright practices are inline with the proposals set out by the Copyright Act of 1790 – he has put version of the site from 1993 until 2003 in the public domain, and adds another year every year. Because links of the books take you to other pages, be careful; some works may be copyrighted, others may be in the public domain.
• Open Library: Open Library is an initiative of the Internet Archive, focused on books. Similar to Project Gutenberg, Open Library instead wants to list every book – every book! They currently offer 1.7 million public domain books in various formats through the Internet Archive.
• Project Gutenberg: One of the more famous and well known public domain sites, Project Gutenberg – founded back in 1971 - is home to more than 56,000 books found in the public domain and available for you. Formats include .epub, .mobi, plaintext, and html.
• Questia: Questia has more than 5,000 public domain, classic, and rare books available – entirely for free. The published date of each book is written underneath the title; if it’s before 1923, it’s in the public domain.
• ReadPrint: Thousands of free books.
• Standard eBooks: A newer and popular entry, Standard eBooks is a volunteer driven project that takes books found on Gutenberg and makes them more readable for modern users. They achieve this with proper formatting and making the text itself easier to read.
• The Baldwin Library of Historical Children’s Literature - More than 130,000 books & periodicals published in the U.S. and Great Britain from the mid-16th century until now are represented here. Some are public domain, but others might still have their copyright active so be sure to check.

Film & Television

• Motion Picture, Sound, and Video Holdings: A part of the National Archives. Much of what is available is free for perusal, but you should reach out to them directly in case you want to use some material for commercial purposes.
• NASA: Because NASA is part of the government, most, if not all, of the media it creates is in the public domain.
• National Park Services B-Roll: The National Park Services B-Roll is in the public domain and free for all.
• Pond5: Pond5 hosts thousands of historic media files, available for free. Film, audio, images, and more. Excitingly, they also have a few videos on works that were created from public domain material.
• Prelinger Archives: The Prelinger Archives are a part of the Internet Archive, started by Rick Prelinger in 1982 to preserve ephemeral films.