The Battle for Wesnoth is a Free, turn-based tactical strategy game with a high fantasy theme, featuring both single-player, and online/hotseat multiplayer combat. Fight a desperate battle to reclaim the throne of Wesnoth, or take hand in any number of other adventures. Easily modifiable. You can even download other users' add-ons from the main menu.
Turn based Strategy, Fantsy World, various Campaigns, MP (server/LAN/hotseat), easy to handle (idiot proof addon manager / good itroduction)
Nice 2D grafics (sorry no 3D GFX)
but grafics make no good game, and this is the best example for it.
First beeing very sceptic about this game, it turned out to be one of the best I ever played, and best of all it is free.
Too due too the fact that it is open source, there are several add ons and sure more to come.
A bit anoying is the dice roll at fights (luck is a bitch), its part of the game, but there is a mod to change that.
I have not checked out the add ons yet its just far too much.