Battle for Wesnoth is a game worth playing not just in light of the fact that it is Open Source, as it is the most outstanding turn based tactical strategy game.
It is a turn based strategy game with beautiful two dimensional graphics, huge world, over a dozen official single-player campaigns and multi-player mode.
The game has a long history since 2005, the year first version was released. It is multilingual and cross-platform. User made content is available at Guide to UMC Content wiki page.
This game is an absolute necessity have for those gamers that are searching for an amusement to play in their spare time, and bear in mind its Open Source.