"Battle for Wesnoth
"Battle for Wesnoth is one of the most popular turn-based strategy games for Linux, if not the most popular. Completely free and open-source, it provides both singleplayer campaigns, as well as online multiplayer, hotseat or LAN support. The game takes place over various maps composed from hexes, and in each hex a unit at a time can be deployed. The player has to take advantage of various terrain types (which increase of decrease defence), use wisely the types of attack and abilities each unit has. Wesnoth currently comes with 5 factions (Loyalists, Rebels, Knalgan Alliance, Undead and Drakes), but the add-on server has a whole bunch of custom eras and new factions.
"SuperTux
"SuperTux is a 2D jump and run Mario clone, with the difference that it uses its own maps, enemies, and the main character is... you guessed, Tux, the story taking place in Antarctica. It is licensed under the GPL and it is free of charge. SuperTux has very nice graphics, especially since it has support for software and OpenGL rendering, 26 levels and 9 types of enemies."
