I usually dislike online games because I don't have enough free time to waste it increasing my level (rpgs) or the action is too fast for my lagged internet connection (first person shooters).
With it's extreme simplicity: slow and clumsy tans and even more slow shots, bzflag is suitable for beginners, but it still includes lots of caveheats that allow the experienced users to get a lot of advantage and become their tanks not so slow, not so clumsy.
Add to it an endless list of game modes and scenarios, and the indispensable powerups and you will get a lot of hours of addictive play.
Good game. This is open source :)
Another game that arrives to the Linux world much later than other platforms. It is a good game anyway, and there are very few similar games available. A good selection of characters, hordes of enemies and lots of action. If you want to kill some orcs and goblins, the huge map of Ancaria is still one of the better places to visit. LGP, when you will port Sacred 2?
Behind it's interface with simple graphics and poor sound you can find and endless challenge where every game is different. It has a huge technology tree, huge maps and a huge amount of options to build your empire and interact with your opponents. I had never spent so much time in a single battle before.
Game: Battle for Wesnoth
Open source games usually lack all those small details that make the gameplay fun. The Battle for Wesnoth is not included in this category. All you may expect to be in a strategy game is here, and it gets even better with every new version. It is a must-play.