RetroAchievements.Org is a cool website that I ran across while configuring my MetaGamerScore.com account. As the site name suggests, this is a gaming site that adds achievements to old school console games.
It currently supports Mega Drive/Genesis, NES, SNES, and Gameboy/Gameboy Advance. The site provides custom emulators that the player downloads and then plays old school games with, earning achievements.
For example, there are 91 achievements possible for playing the excellent GBA game Advance Wars.
Not every game is represented, but it appears they are actively adding new games regularly and taking suggestions for both games and/or achievements.
Still, very cool.