Free

Attal: Lords of Doom
Turn-based strategy game for Linux in which you control bases and manage fighting lords.
http://attal.sourceforge.net/

Babala
Logic game using mines to defeat computer controlled players. Includes client download and instructions with screenshots.
http://www.babala.org

The Battle for Wesnoth
A turn-based game with a fantasy theme. Includes download, project information, FAQs and manual, screenshots, and a forum. (Linux, Mac OS X, Windows)
http://www.wesnoth.org/

Boa game
A multi-player clone of the worm(6). Includes, server, client, and bot. Source can be compiled for SGI, Linux, Sun or Gnuwin32.
http://www.tarunz.org/~vassilii/pub/boa/

Boson
A real-time Linux strategy game for two players. (Open Source, GNU GPL)
http://boson.eu.org/

Castle Combat
A clone of Rampart for Linux. Two players build castle walls, place cannons inside these walls, and shoot at the walls of their enemies.
http://user.cs.tu-berlin.de/~karlb/castle-combat/

C-evo
Turn-based empire building strategy game for Windows. (Open Source)
http://www.c-evo.org

Civil
A cross-platform, turn-based, networked strategy game, developed using Python, Pygame and SDL for scenarios set during the American Civil war. [Open Source]
http://civil.sourceforge.net/

CivQuest
Turn-based Civilization clone written in Java. (Open Source, GNU GPL)
http://civquest.sourceforge.net

The Colonization II Project
Project to create an alternative sequel to the turn-based strategy game Colonization by Sid Meier.
http://www.civgaming.net/forums/forumdisplay.php?s=&forumid=14

Constellation!
Constellation is a 2D turn-based pure-multiplayer strategy / combat game for Windows with a space theme. Well suited to be played by email.
http://groups.msn.com/Constellation/_whatsnew.msnw?pgmarket=en-us

Continents
Turn based sprites with random map generation and sound.
http://www.geocities.com/vb_user/

Crimson Fields
Describes tactical war game for Linux or FreeBSD in the tradition of Battle Isle. Two players command their units on a map of hexagons, trying to defend important locations to destroy all enemy forces. (Open Source)
http://crimson.seul.org/

Crown and Cutlass
A 3D pirate game in the spirit of the old Pirates! game.
http://crownandcutlass.sourceforge.net/

Cult
Using negotiation and trade, one must resolve a situation involving cultists barricaded in a villa two kilometers away from town. (Windows)
http://studio-blum.com/Cult/

Emperor of the Fading Suns: Hyperion
A patch by Matt Caspermeyer for the turn-based game. Includes download, documentation, tutorials, and a forum.
http://hyperion.twarriors.com/

FreeRails
Multiplayer real-time strategy game inspired by Railroad Tycoon and Railroad Tycoon II. Written in both C++ and Java. (Open Source, GNU GPL)
http://freerails.sourceforge.net

The General
Turned based military strategy game for Windows in which you control a nation. Your goal is to unite the world under your rule, upon conquering all the other states.
http://newgame.agava.ru/eng/g_news.html

Glest
Single player, 3D, realtime strategy for Windows with up to three computer opponents.
http://www.glest.org/

L-O-C
Turn-based strategy game with some strong RPG elements, based on a game by Julian Gollop. Runs under MS-DOS, Windows and Linux by using an Atari emulator. (Open Source, GNU GPL)
http://l-o-c.sourceforge.net

Marbles
Create figures out of single marbles. One-way streets, crumbling walls and portals make the task interesting. (Linux)
http://lgames.sourceforge.net/index.php?project=Marbles

Overlord '44
Overlord '44 is a 3D real-time strategy game about the D-Day landings. You will command either the Allies trying to get further inland or the Axis trying to repel the Allies.
http://www.freewebs.com/overlord1944/

Pizza Business
Control pizza restaurants, limited to a budget. Includes downloads and news.
http://pizza-business.sourceforge.net/

Planetasia
Space strategy game for Windows.
http://planetasia.freecyberzone.com

Pocket Civ
Turn-based Civilization clone for Palm OS.
http://pocketciv.radioxy.com

SantaGames.co.uk
Contains "Santa's Snowy Workshop" available for download. [Windows All]
http://freespace.virgin.net/tim.wintle/santagames/

Scorched Island 3D
Combines tactics from popular game Scorch and elements from RPG games for DOS or Linux.
http://cvs.linux.hr/si3d/

Trackballs
Marble simulation based on the 80's classic "Marble Madness". You steer a marble through obstacles to finish level on Linux or Windows. [Open source, GPL]
http://trackballs.sourceforge.net/