Images, audio, video

Awesome Stock Resources: a curated list of stock photography, video and illustration websites
Wikimedia Commons: a large database of freely usable media files (images, sounds, videos)

ImageNet: image database organized according to the WordNet hierarchy (image-net.org)
Open Images: dataset of ~9 million URLs to images that have been annotated with labels and bounding boxes spanning thousands categories (github.com)
Flickr: CC-licensed content for commercial use on Flickr
Unsplash: free high-resolution photos, also available via Unsplash API (also see documentation)
Random Image Generators (thisiskelv.in)
11k Hands: a large dataset of hand images

Museum of Modern Art Exhibition History: from 1929 to the present
The Metropolitan Museum of Art Collection
New York Public Library Public Domain Images
The Behance Artistic Media Dataset: Automatically-labeled binary attribute scores for over 2.5 million images across 20 attributes each, and more (bam-dataset.org)
National Gallery of Art Images: 35,000 works of art from the National Gallery, including masterpieces by Van Gogh, Gauguin, Rembrandt, and more
Smithsonian: 40,000 works of Asian and American art, can be used for all non-commercial purposes
The Morgan Library & Museum: Rembrandt Prints

googlecreativelab/quickdraw-dataset: collection of 50 million drawings across 345 categories, contributed by players of the game Quick, Draw!
Old Book Illustrations: illustrations from old books; this very site uses it extensively!
Clipart: free clipart images
The Digital Comic Museum: public domain Golden Age Comics
Amazing Stories: the world’s first science fiction magazine, launched in 1926 (archive.org)
U.S. Department of Agriculture (USDA) Pomological Watercolor Collection: 7,584 watercolor paintings, lithographs and line drawings, including 3,807 images of apples
- @pomological tweets these images randomly
Reusable simple drawings (thedoodlelibrary.com)

David Rumsey Historical Map Collection: over 67,000 images of 16th through 21st century maps of America, North America, South America, Europe, Asia, Africa, Pacific and the World (davidrumsey.com)
State Library of New South Wales: Open Data: see the DATASETS tab for maps, posters, and more

Project Apollo Archive: more than 8,400 high-resolution photographs taken by Apollo astronauts during trips to the moon
Wellcome Images: a medical picture library and the world’s leading source of images on medicine and its history
ECUSTFD: a food image dataset (2900+ images) for evaluating computer vision-based methods (github.com)
- Computer vision-based food calorie estimation: dataset, method, and experiment (arxiv.org)
udacity/self-driving-car: image frames from car-mounted cameras, and information about latitude, longitude, gear, brake, throttle, steering angles and speed

Visual Geometry Group: Oxford-IIIT Pet Dataset: a 37 category pet dataset with roughly 200 images for each class (robots.ox.ac.uk)

These are not always downloadable, but you can at least freely link to them.

AP Archive: the film and video archive of The Associated Press
Kinetics: a large-scale, high-quality dataset of YouTube video URLs which include a diverse range of human focused actions (deepmind.com)
Beachfront B-Roll: HD stock video footage and animated backgrounds
mazwai.com: a small collection of stock videos
coverr.co: more stock videos

British Pathé: historical footage
British Movietone: newsreel archive, spanning the period 1895 to 1986
The Local News: An Archive: a large collection of samples of local news programs and broadcasting styles over the decades (archive.org)

AudioSet: A large-scale dataset of manually annotated audio events
The NSynth Dataset: Large-scale and high-quality dataset of annotated musical notes
freesound4s: 126,900 sounds on Freesound less than 4 seconds long
- see also: Fingerprints and Labels to Classifier.ipynb
IMSLP/Petrucci Music Library: Free Public Domain Sheet Music (imslp.org)
Old Time Radio Researchers Library: more than 85,000 individual episodes from nearly 2,000 series
BBC Sound Effects Archive Resource: a collection of 16,000 sound effects (bbcsfx.acropolis.org.uk)
BitMidi: 100,000+ MIDI files curated by volunteers (bitmidi.com)