RGB Classic Games Forum

Recently I've stumbled upon this really huge database of free games of all flavours (quite handily split into categories):
Megazell's Free And Legal PC Games List

I'd say it's easily the most comprehensive list of all I've encountered so far. Links to other free gaming websites are also available.

Also, some other websites to check out:
http://libregamewiki.org/Main_Page
http://osgameclones.com/
http://planet.freegamedev.net/
http://www.reloaded.org/
http://www.caiman.us/
http://www.curlysworldoffreeware.com/
https://help.ubuntu.com/community/ListO ... rams#Games

Here's a few more nice gaming websites (mostly indie but not exclusively) with reviews, links and freeware, shareware & demo downloads:
ManifestoGames
Play This Thing!
Rampant Games

Free Games Directory at Uziana.com, a review site for "rare and alternative" games.

I think I haven't mentioned these two yet (although I fancy they are pretty well known):
Retro Remakes
Freeware Database - Retro Remakes