I don't know how this game isn't more well known, but I only just found out about XEvil. This game is a cross between Commander Keen and Quake and it's very fun, if a little dated. Action packed and chaotic, it's all about extreme violence platform-style. Be warned, this game contains lots of violence and lots of adult language so really is not one for your kids should you have any.
I'm keeping an eye out for the next release of Privateer Remake or Privateer Gemini Gold. In the latter they are busy taking the rather garish low-res base and npc graphics and overhauling them, so the next release should be much more pleasing on the eye. When I downloaded PGG, I found the low-res bases too much of a clash with the gorgeous 3D space flight and it put me off considerably.
There's also a new release of Battle for Wesnoth around the corner. The amount of people contributing to this game is phenomenal and as such version 1.2 will look absolutely gorgeous. There's a massive amount of graphical polish going into the 1.2 release - lush new terrain graphics, superb portaits, and many unit updates - and a lot of new features in the game that will make it an even more pleasurable experience.
Also scheduled for a newer release is Boson. Whilst the release plan seems to have slipped a little, this is one of those open source games which has just chugged steadily away, incrementally improving without really setting alight the open source gaming community. It'd be nice to see them focus on the single player element a bit more because I think people are generally fed up of having to find opponents and engage in constant deathmatch style competition.
My opinion is that single player games are probably more valued by players, but the industry MMORPG / deathmatch polarisation has tilted development goals needlessly in favour of multiplayer gaming. I like to play a game casually to unwind, not get whooped by some ultra competitive online obsessive!