Abandonia is a clearinghouse for abandonware: software which is no longer sold or supported. These are typically games made for systems which are no longer on the market. But thanks to emulators, they're still playable.
The site is Mecca for old-school computer gamers. All the Infocom games are here for free, for instance.
I'm still looking for a playable copy of Atari's BallBlazer. Lemme know if you find one.