
BZFLAG
Runs on Nix and Win32
You control a tank with a difference....it can jump! Other tanks are controlled by other players over the internet and all connect to a BZFlag server. You can even setup your own server if you wish.
Whoever runs the server can choose various features that effect the game eg. jumping, bullet ricochet, flags which can be collected, etc.
There are two modes of gameplay, "free for all" and "capture the flag or CTF". Free for all can be setup to include four different colour teams and also to allow rogue tanks. In CTF, once you return to your base with an opposing team's flag, your team gains a point and the opposing team's tanks are destroyed.
Teleporters are normally dotted around the map which, when entered, can transport you to another teleporter, let you pass straight through them or send you back in the direction you entered. You can even shoot into a teleporter but be careful it doesn't bounce your shoots straight back at you!

BZFlag links:
BZFlag online gaming stats & map posting/development
DESCENT III (D3)
Runs on Windows 9x/NT 4.0 (SP3+), Mac and Linux
This is a commercial game but is bundled with certain graphics cards. I got mine with a Matrox G400 dualhead!
The game is in the FPS (first person shooter) genre, which means that your perspective is through the eyes of the computer generated 'person' that you control. In D3's case, the pilot of a spaceship.
Unlike practically all other FPS games though, in D3 you have complete control of movement across all axis! This is one of the unique aspects of the game which was first introduced to the FPS scene when D1 was released. It can be quite disorientating for the new player but as they say, practise makes perfect!
Solo gameplay is good and there are many levels with various problems to overcome but the best part of this game is playing against other players over the internet.
There are various modes of online gameplay. Anarchy is...blowing the hell out of all other players, Team Anarchy speaks for itself and CTF, or capture the flag, is a team based game where the objective is to capture the opposing team's flag from their base whilst preventing them from doing the same to your flag.
Other modes of online gameplay are available but these are the most common.

D3 links:
Interplay's "The Official DESCENT 3 Web Site" but the main focal point of the D3 community is at PlanetDescent.
Stuck playing D3 and don't know what to do?! Check out GameSpot's Descent 3 Game Guide
Outrage Entertainment The creators of D3.