The Battle for Wesnoth is a pretty sweet turn-based strategy RPG game. It had been at version 1.0 for the past year and the development team just announced the release of version 1.2. It's available for Linux, Mac and Winows. It's a great example of community effort with its artwork and character animations, and provides solid gameplay provided by the development team.

There are also 20-30 community-developed campaigns in addition to the official 2-3 campaigns. It's a fun game, check it out: http://www.wesnoth.org/