Here are the tiles used in the game Dungeon Crawl Stone Soup http://crawl.develz.org/wordpress/

Package contains over 3000 tiles! This is a collaborative effort from many, many artists. The full list of artists can be found in the package crawl-tiles Oct-5-2010\utility\FilteredList.txt

All images are 2D 32x32 pixel tiles in orthogonal (overhead) three-quarters  perspective. 

Contains things like terrain, walls, decorative dungeon features, monsters, spell effects, items, GUI elements, player avatars, and more!

EDIT: A new supplemental set of over 3000 additional tiles is now available! Check out https://opengameart.org/content/dungeon-crawl-32x32-tiles-supplemental