This is also the first time I have used our new multi-page article system. Enjoy 3 pages of open-source glory.
I saw someone state we didn't cover open-source enough, so here you are!
This is by no means a full list, just some favourite projects of mine that I think deserve a little attention from GOL visitors. Hopefully this can spur up a good conversation in the comments on some of your favourites to share with us too!
Featured
To start it off with one I have never mentioned before!
Knights Game
This game will always have a place in my heart and it's probably not very well known, I played the original on Amiga CD with my brother and it's now open source and available for Linux! Each game revolves around a quest – for example, you might have to find some items and carry them back to your starting point. This may sound easy, but as there are only enough items in the dungeon for one player to win, you may end up having to kill your opponents to get their stuff!

Seven Kingdoms
Seven Kingdoms Ancient Adversaries was open sourced under the GPL in 2009. This includes the game source code, graphics, and audio, except the music.
Another project that I have fond memories of, I originally purchased this when I was younger as part of a double game pack with Deadlock 2 and was pleasantly suprised by this awesome Real Time Strategy with some really different game play!

You must control as many villages as you can as they are your resource for getting troops, while fighting or making friends with rival kingdoms.
0 A.D. - A Real Time Strategy engine and historical game with beautiful graphics, this game releases an Alpha version a few times a year usually with lots of bug fixes and new features to test out, still far from a playable game due to poor perfomance as a game goes on, but they are working on it!

I fear though that 0 A.D. will end up in a perpetual Alpha release state, while they have interesting additions with each new release I can't help but feel like it will just go on and on with no stable release.
OpenXCom - Open source engine for the first XCom game XCOM: Enemy Unknown (UFO Defense in the USA) a Turn Based Strategy game.
This is one of the many reasons I truely love open source, this game is probably one of my all time favourite games that has been revived thanks to this project!
SpringRTS - Open source Real Time Strategy engine originally meant for Total Annihilation but now has a vast array of games for it, games like Zero-K for example featured in the below trailer. Sadly their player count seemed to fizzle down quite low since some time ago.

GemRB - A portable open-source implementation of Bioware's Infinity Engine, think being able to play Balders Gate or Icewind Dale on Linux natively. I seriously need to give this a go, I loved Balders Gate 2.

Take a look at page 2 for more.

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