Probably one of the best and most widespread inadvertent ads for BYOND, long ago BYOND's own nick.cash dealt a blow to unfinished 2d games everywhere and published a link set of massive proportions to free tilesets all over the web.
If you don't find it here, expect to pay some kablinki, my friend.

[EDIT:] And now that I come to think of it, a lot of the free 2d resources any of us will find out there and post here will more than likely overlap with nick's list, but that shouldn't stop us.