A good open source platform game that never gets a mention is The Goonies: 20th Anniversary Edition. This game is a remake of the classic NES / Master System version by Konami. The same team is also responsible two rather fun arcade top-down racing games, F1 Spirit and Road Fighter, which are also remakes. I'm not sure what the source license is for these (ack, there goes my Free Software credibility) but you can get the source and the games compile / run on the 3 major platforms (y'know, Lin/Win/Mac).
There's not-so-recent updates to Daimonin, the 2D isometric fantasy MMORPG that seems to play second fiddle to the likes of Eternal Lands and Planeshift due to it not coming with the fancy 3D 3Dness. I mean, everybody wants 3D, and those who don't play nethack. So there's no room for anything else. Especially not something as addictive as Daimonin. I'm not going near that game again, I barely escaped the first time! Anyhow, it's up to version 0.9.7.1 which means something to somebody, somewhere.
Egoboo has been travelling, but after several years finally returned home to egoboo.sourceforge.net where it's open source adventure originally started. Development is highly active, with lead developer Zefz having dragged it kick and screaming back from the depths of stagnancy. (My god, that's actually a valid [ab]use of the word... I mean, I typed it as a joke, and the spell checker didn't cry.)
Entropy found via the Ubuntu forums, is... I'll use their words, "Entropy is an open source game project the likes of which you have never seen. With what is currently a small team of modelers, texture artists, and programmers, development on Entropy (which just finished the final stages of design) is... brand new and revolutionary, in both its spirit and style of gameplay. Entropy will be a structured mix of First Person Shooter combat, with vehicles of all sorts, combined with Real Time Strategy, with a few Role Playing elements thrown in. However, the most important aspect of Entropy is that it is entirely online and immersive, almost every character in game will be an actual player."
Sounds very interesting. However, I believe their ambition far outweighs their ability to deliver on it, something I expressed. However, they are 'veterans'. Whether being veteran means 1 guy can program a commercial grade game in 2 years when programming Gods (think John Carmack) don't do that, well, who am I to be a party pooper. Still, I hope they succeed.
All I'll say is that, unless you have fairly modest requirements, programming your own engine from scrach is absolute madness with the likes of Ogre3D (and the plethora of other options) at your fingertips. (Note: we need to make a page for these on the FreeGameDev wiki.)