Some other interesting strategy and simulation games:

<span class="bold">FreeCol</span> - fully playable Colonization clone, but gets somewhat slow the longer you play
(Java based, may work well on faster computers)
<span class="bold">Widelands</span> - an improved Settlers II clone
<span class="bold">Unknown Horizons</span> - a simulation game similar to Anno 1602
<span class="bold">7KAA</span> - the open source version of Seven Kingdoms
<span class="bold">Megaglest</span> - a 3D RTS game, somewhat similar to Warcraft 3

Wesnoth is a really good game.

EDIT: 0ad- https://play0ad.com/ Have not played the recent updates. played it a long time ago. But it seemed to hold great potential.

Dungeon Crawl Stone soup
DoomRL
A.D.O.M (Ancient Domains of Mystery)
Transcendence

freespace 2 with wing commander saga.

Liquid War 5 is kinda fun! And Scorched Earth 3D

Why no one know Endless Sky?

I thought it was pretty well recognized in the open community

It's a good space sim game where you actually Must think what kind of ship you want, it's not just push up better MK version, and the story is decent with some possible variations depending on player choice, relatively easy to mod as well

Only issue is that some parameters are hard coded, and to change them you need to recompile the whole game, given what the developer used, it's not an easy task on windows, despite various repeated attempt i was unable to make it compile, but the game work fine and most settings are moddable

Warsow, an arena FPS on the spirit of (and based on) Quake 3 but even faster, it's even available on GoG.

Battle for Wesnoth, a fantasy TBS with RPG elements.

Dungeon Crawl: Stone Soup, a roguelike with a faster pace and a (slightly?) gentler difficulty curve than Nethack.

And of course, no such list could be complete without...

Dunnet, a really, really out-of-the-left-field text adventure game, which comes bundled with the Emacs editor; just execute M-x dunnet to play.

OpenArena - ClanArena and CTF modes.

Zandronum - absolute fun with Ukito servers and RGA2-modded servers.

Gunroar - for being awesome top-down shooter (I play native linux port).

Call of Chernoby - (its actually opensource, based upon opensourced Xray engine)

CnC: Rise of the Reds - best CnC mod ever (mod is open, but core game is not, here only because of being exceptional)

Is it already working? Last time I checked not much had changed so I gave up looking.

Battle for Wesnoth is by far the open source game I played most.

Yes, I finished one game with last version and new version added fighters and carriers. In many ways it already beats Master of Orion 3 but things like trade is still totally missing. A little bit of diplomacy has been added too. Version 0.4.7 is the current one. For Linux you will need to compile yourself unless you can find soneone you trust who has already done so.

SuperTuxKart

+1
Fun game. Used to play it a lot.

Xonotic, it's just a really damn good FPS game with the Quake movement and weapons with Unreal Tournament inspired alt-fires.

FreeDink, best way to play Dink Smallwood, especially for mods and custom adventures. Dink Smallwood HD is only good for playing the original and besides, this source port has an actual working joystick and DinkHD's fast forward button.

Hmm, there were the Cube engine games too but I forgot the names.

Ah, here they are: http://cubeengine.com/
so:
- AssaultCube
- Cube
- Cube 2: Sauerbraten

TAGAP, TAGAP 2, Freedroid.