Turn-Based

The Battle for Wesnoth
A turn-based single and multiplayer game with a fantasy theme. Manga-like graphical style. Includes download, project information, FAQs and manual, screenshots, and a forum. (C++) [Linux, Mac OS X, Windows] [GPL (code and content)]
http://www.wesnoth.org/

Civ II Evolution Project
Developing an Open Source strategy game based on Sid Meier's Civilization II. Public project.
http://c-evo.org/

UFO Alien Invasion
UFO:AI is a 3d turn based strategy game inspired by the X-Com series by Microprose. (C) [Windows, Mac OS, Linux] [GPL2]
http://ufoai.sourceforge.net