Battle for Wesnoth
Well the big release of the weekend is Battle for Wesnoth 1.4 (changelog). Not too much to say about this game that hasn't been said hundreds of times before. It's simply the best open source game project going. Hundreds of contributors, graphics rivalling contemporary commercial 2D games. Over version 1.2, Battle for Wesnoth 1.4 brings 7 new campaigns, major UI improvements, major multiplayer improvements, many improved graphics (the portraits are stunning) and many bug and balance fixes.
Wesnoth is so popular that the response to this release seems to have overwhelmed parts of their website. The wiki and forums are unable to cope.
I'm wondering where they will draw the line with this project though. The main developer has since moved on to SilverTreeRPG. Obviously they could keep adding campaigns, refining the graphics etc, but surely at some point you want to say "this is it, this is the final v1 of Wesnoth" and look at making v2 which is a significantly different game (be it improved or different storyline or whatever). They surely just can't keep on evolving this version because then, well, it seems to be a bit of a waste of resources that could be used to create a different version. Once a game is complete, development is of diminishing returns. Sure, you can make it a bit shinier, but it's of much less value to the game playing community who [I think] would rather see major strides made on a new game rather than the same game have relatively small improvements.
OpenFracas 0.4 (currently just for Linux and Windows), "a free, open-source game that is similar to Risk." There's more gameplay features than in standard Risk, and it's also well done, so check it out if you like that kind of strategy game.
SuperTuxKart 0.4 is now up for download. No screenshots of 0.4 on the website, although it looks the same as 0.3 for the most part.
Space Exploration v5 - a fun little 2D space exploration and trading game. This version greatly enhances the UI, but I think the Java detractors will love this version because it's very very slow. I suspect the author isn't too experienced on Java graphics programming and as he learns more it will speed up greatly. Anybody who believes Java is slow should try out the rather spiffilicous JCRPG which not only looks beautiful but is very fast too. A bit of perseverance and Java "force" is all that's needed.