The Battle for Wesnoth is a turn-based strategy game. There is very few games of such quality among open-source ones: though it looks dated, and there is plenty to improve, it uses a sane build system and not too many dependencies, making it easy to install if it’s not available in your system repositories. It’s also written in C++, and should run on many systems, beyond Linux and BSD.
The game is quite simplistic in some aspects (say, gold is the only resource there), yet fun and challenging – perhaps because of the battle mechanics that are not that simplistic: some units fight better at day or at night, many of them are affected by terrain type, they level up, there always is randomness, and a bunch of other things. The graphics are fine, for it doesn’t try to do anything too fancy: just sprites with appropriate graphics, and more of occasional art to accompany cutscenes. Nice music, too.
Fantasy theme is used, featuring orcs, elves, humans, and other factions. The setting is not particularly original or outstanding, yet it’s done nicely.
There is a variety of scenarios (or campaigns), so one can easily kill a lot of time with it.
Here is how it looks like:
The Battle for Wesnoth screenshot
Overall, the game is well worth trying.