A little different from many of the things mentioned, but...

• Tales of Maj'Eyal - an open source Roguelike with a ton of content. There are paid expansions but the engine and base game are FOSS.
• 0 A.D. - AOE-like
• Battle for Wesnoth - a really fun TBS
• gzdoom + freedoom - while the assets aren't quite on par with the commercial Doom assets, this will allow you to play through any Doom mod/TC