The Battle for Wesnoth

From developer's site

What is Battle for Wesnoth?

The Battle for Wesnoth is a turn-based strategy game with a fantasy theme.

Build up a great army, gradually turning raw recruits into hardened veterans. In later games, recall your toughest warriors and form a deadly host against whom none can stand! Choose units from a large pool of specialists, and hand-pick a force with the right strengths to fight well on different terrains against all manner of opposition.

Wesnoth has many different sagas waiting to be played out. Fight to regain the throne of Wesnoth, of which you are the legitimate heir... step into the boots of a young officer sent to guard a not-so-sleepy frontier outpost... vanquish a horde of undead warriors unleashed by a foul necromancer, who also happens to have taken your brother hostage... guide a band of elvish survivors in an epic quest to find a new home.

200+ unit types. 16 races. 6 major factions. Hundreds of years of history. The world of Wesnoth is absolutely huge and limited only by your creativity - make your own custom units, compose your own maps, and write your own scenarios or even full-blown campaigns. You can also challenge up to 8 friends - or strangers - and fight in epic multi-player fantasy battles.
Features

• Build up a formidable fighting force, starting from a single leader and a small amount of gold.
• Over 200 unit types in six major factions, all with distinctive abilities, weapons and spells.
• Experienced units gain powerful new abilities as they advance.
• Several multi-player options available, including internet play.
• Scores of different custom-designed maps, and unlimited random maps.
• Hundreds of campaign scenarios available for download via a simple in-game procedure.
• 'Fog of war' feature available for a true test of generalship.
• Sophisticated mark-up language lets advanced users make their own maps, factions or campaigns.
• Excellent language support 35 different languages currently available.
• Windows, Mac OS X, GNU/Linux, RISC OS, BeOS, Solaris, FreeBSD, OpenBSD, NetBSD, DragonFly BSD, AmigaOS4, OS/2 & eComStation compatible.