Artists For Hire
http://forums.indiegamer.com/forumdisplay.php?f=20
Reiner’s Tilesets
http://reinerstileset.4players.de/englisch.html
Lost Garden
http://www.lostgarden.com/search/label/free%20game%20graphics
Gamedev
http://www.gamedev.net/topic/272386-sprites-sprites-and-more-sprites/
SpriteLib GPL
http://www.isk.kth.se/~johanseg/files/prgprj/grafik/spritelib_gpl/