Useful for games, blog posts, articles etc:
Flickr lets you search specifically for Creative Commons-material, mostly photos.
Wikimedia Commons has a huge repository, a lot of which is historical.
Internet Archive Book Images, huge volume of book illustrations in the Public Domain.
DeviantArt: Do a site-specific search on Google Image Search. E.g: «owlbear site:deviantart.com»
Select Search Tools -> Usage rights. Select, for instance, «Labeled for noncommercial reuse». (Still turns up a lot of irrelevant results, but I guess it’s better than nothing).
http://fossilbank.wikidot.com/ has a bunch of links to free/open licensed artwork.
Thanks: Yanni C, Nick W.