The older version of this topic is getting quite dated, with some broken links and notable omissions, so after 8 years it's high time for an update.
This is a list of free graphics for games but aims to avoid sprites ripped from existing games in favour of ones that may be legally used in your work. Please feel free to submit your own suggestions, but note that any off-topic posts and all spam may be removed from this topic.
Be sure to check the licensing terms before using any of the linked graphics.