Battle for Wesnoth: Turn-based fantasy and strategy adventure
The Battle for Wesnoth is a turn-based strategy game with a fantasy theme where your task is to build a great army in order to regain the throne of Wesnoth. If you've ever played Dungeons and Dragons then you'll be familiar with the sort of territory that The Battle for Wesnoth covers. It's surprisingly complex and not a fantasy game that you can immediately get into but The Battle for Wesnoth has many treats hidden in its depths.
For a start, the amount of roles you can play gives the game great variation. Regaining the throne is but one scenario - you can also guard a frontier outpost, take on a load of undead warriors and guide elves to make a new home in the kingdom. There are more than 200 units to take control of with 16 different tribes and six major factions. And if that's not enough, you can even create your own maps, scenarios, and unit types. This ability to create your own style of game is really impressive for an open-source RPG.
The Battle for Wesnoth is also a highly tactical game, requiring you to move units around in order to outfox your enemies, which takes some doing with several units on the go at once.
As far as open-source RPG fantasy games go, The Battle for Wesnoth is an excellent effort.