more like inspiration but this website has tons of game concept art to inspire and amaze! http://www.creativeuncut.com/game-art-galleries.html and this site documents artworks from different cultures all through history http://www.metmuseum.org/collections/search-the-collections?ft=Assyrian&who=Assyrian&ao=on
Calculators that help you calculate the time when its safe to leave the vault and other stuff in Health Physics Historical Instrumentation Museum Collection
http://invisiblethreads.com/potd/collections/galleries.php?cc=1 dunno if posted before or not. just a bunch of images of urban decay in and around ontario it seems.
Extensive collection of slow motion videos, shot by high-speed motion-picture camera. Very useful for particles and dynamics simulations. http://www.lucidmovement.com
Haven't seen this posted in the last year, so: http://www.googleartproject.com/ Massive collection of artwork from musuems across the world. No way to save but good ole printscreen and manual compositioning, but definitely worth the effort.
@McGreed I also have trouble with that. Using folder structures gets very complex and many references can go in different locations. I looked up some ways to create tags, like collections in Adobe Lightroom (clumsy work to tag whole folders and create collections) or various software (hard to search, bad interfaces) but…