Favorite Free Software/Open Source games?

I just gonna put this here. Maybe somebody is interested.
tiggit.net

Free Game = FOFiX. Frets on Fire is a Guitar Hero clone thats been out for years and FOFiX is more a Rock Band clone that includes drums and fully customisable controls so you can use real RB or GH instruments if you wish but just a regular keyboard will suffice.
I'll leave some links and if you like GH or RB then this is a must as you can find SO many songs on the forums.

FOFiX download
Rock Band Theme - Sorry you'll have to look around if you want it because it appears the original file I got has been removed.
And finally if you want to look for a song in The Forum Just make an account on the forums and all the songs are belong to you, more or less.

Haven't explored the topic much, but FreeCiv is a solid 4X game.

Also, FreeOrion looks promising, however it is still under development. Even started to be more enjoyable for single player, when they started to work more on AI, otherwise it was a bit sandboxy (just building a cosmic empire without much opposition).

Not exactly a recommendation, but if someone's interested in the topic, Wikipedia to the rescue with its list of open-source games. One of these days I wanna try Dink Smallwood, for instance.

I've played many good, free games, but only recently I discovered Tales of Maj’Eyal (ToME), which is probably the best Roguelike I've played thus far(and usually I can barely play those). highly recommended: http://te4.org

Hedgewars

FreeCiv, Wolfenstein: ET, Quake Live and the old Nexuiz.

Frets on Fire, HackNet(not sure if it's open source), Notepad++, GIMP, Paint.NET(don't knock it! It's almost as good as GIMP), Katawa Shoujo and Gang Garrison 2.

Firefox and forks.

Gravity Bone

Toribash is a game i tend to go back to whenever i'm bored, its a ragdoll fighting game, where you control every muscle in the character, and can do INCREDIBLE things, considering not only gravity is present, but also momentum, inertia from the movement of other limbs and muscles, aswell as the posibility to break them, cut limbs and grab. Its turn based, has a great modding community, tournaments are held every now and then. It also has VERY big difficulty curve, but once you get the hang of it, its extremely satisfying.

Also, Frets on Fire (Guitar Hero clone written in Python) was a huge addiction for me a couple of years ago.

Maybe Urban Terror. You should give it a try.

Tyrian 2000, which happens to have quite a few source ports, too.

OpenTTD

Jagged Alliance 2 v1.13. Yes, it's an open source game/mod, but you need the original game assets :)

Ur-Quan Masters and Spelunky are both completely amazing.

Better than most games people pay for anyways...

Cavestory

Nitronic Rush: http://nitronic-rush.com/

Deity: http://www.deity-game.com/

Also, this might help: http://en.wikipedia.org/wiki/List_of_open-source_video_games

Dwarf Fortress is a very fun game, and Spelunky also looks fun.

And even though its not free, Freespace 2 is open source. There are some awesome mods/total conversions/full games that are worth checking out. Like this

Yume Nikki.

Powder, i like to make stuff explode.

Eternal Silence, HL2 mod. Don't know if mods count, but no one plays it anymore anyway. makes me sadtimes.

Nation-states.... A political sim, loosely based on some poor shmuck's book. He helped design a helluva fun time waster.

Face of Mankind

Chromium B.S.U.

no liero
for shame