Video Game Sprites:
http://www.videogamesprites.net/

Panel Monkey:
http://www.panelmonkey.org/

The Sprite Database:
http://sdb.drshnaps.com/

Spriters Rescource:
http://spriters-resource.com/

The Mystical Forest Zone (or MFZ):
http://www.themysticalforestzone.com/