Minetest.

One of my favourite games which was originally a commercial game but has been OSS for a long time now is crack.com's game Abuse.

The Dark Mod is a gem. It is a game which is heavily inspired by the Thief-series and runs on the Doom 3 engine (however it is stand-alone and require nothing), if my recollection is to be trusted. It is a first-person stealth game in a gothic steampunk world and you play as a thief with access to different weapons and tools. The voice acting is not worth mentioning, it is rubbish, but its "olde english" language has to be praised. There's no story though, instead you get to play and choose from a vast collection of missions which are downloaded when chosen.

I highly recommend this.
http://www.thedarkmod.com

I couldn't say what's my favorite but this site: http://osgameclones.com/ is a good place to start at. I can't say if it's complete but it is has a lot of info.

Doom 1&2 - map packs for years, baby B).

No one mentioning Jagged Alliance 2? I'm disappointed.

Battle For Wesnoth is an amazing game.

I would've definitely if I knew it was open source. It's an incredible game that I've still haven't managed to finish (probably because I restarted when I discovered Arulco Revisited).
Are you talking about the basegame though or the 1.13 mod?

Source was given to the public with the Wildfire release. Assets are not public, but the source is. 1.13 is a product of that open code.

Ah ok I see that makes sense.

Knowing that, if only I had the know-how I would dedicate myself to improving the way character graphics work: at the end of the game you are dressed from head to toe with cool army gear but you still look like you did when you started the game. As is the only way to make a merc look cooler is to wear enough camouflage :\

I did not know it too. It's even available from the Debian repository. Looks interesting... and I already died. :P

Thanks for the recommendation!

Debian has it in it's repository, somewhat behind though (0.4.5 and 0.4.6 in unstable), as usual. ;)

I still would make a difference between open source games and open source engines and only would call it an open source game when all components needed to play the game are open source.

0.4.5 I would not want to play as it still had performance problems, meaning processing turns could take a long time.