So, where to start?
Maybe with some nice puzzle games: Rezerwar is a pretty nice mixture of a pipes game and some sort of tetris, and it was even created using only open-source software. Another pipes game is Cooldown, which might be worth to check out, too. Graphically a bit more advanced is Octaspire: Crates, a block movement type game or something like that ;) But see for yourself:
A non puzzle game worth checking out are Hero of Allacrost (a 2D RPG), which recently had it's DEMO 1.0 release. Another, albeit 3D RPG (engine) is DNT (DccNiTghtmare), which is set in a "satirical post-apocalyptical world".
If you are following the development of the promising turn based colony game Unknown Horizons (aka the artist formerly known as OpenAnno), their new development blog might be of interest. Oh and did I mention that FreeCol also got a bunch of updates lately?
Very neat is also the recent release of FreeOrion, a turn based space colonization game. It still lacks some important features to be really playable (combat and diplomacy), but otherwise it is already a pretty good looking game:
Another game currently heavily updated game (engine) is OSARE, which aims to be a fully featured base to build Diablo like story lines/games with. An already slightly outdated (v0.08, newest v0.09) shows what can be done already:
Another interesting project I came across lately is the Darkplaces based Blood Omicide, which is of course sadly based on copyrighted material. Overdose, a recently already mentioned open-source "in engine but not in media" (and even the engine code has yet to be released) game has recently shown some rather nice enemy model screen-shots. Furthermore their newest video is quite amazing in regards to the light rendering.
A game-engine and the games based on it that is probably vastly under-reported here on FreeGamer, is the Spring RTS engine. I guess it might be due to my dislike of Total Annihilation like RTS, but there is actually one (with public domain licensed media) that strays from that kind of RTS concept: Kernel Panic puts you right inside of a PC, and has quite a bit of nice uncommon game modi that break up the usual RTS grind.
I guess that's all for today... my link-list is not yet completely empty, but the rest will have to wait for another day ;)