Stargem already mentioned it, but I have to reiterate:
Battle for Wesnoth
Available for Mac, Windows and Linux, Battle for Wesnoth is a turn-based strategy game somewhat like Fire Emblem meets Ogre Battle meets Warsong. It's easy to learn, but is challenging and doesn't baby you like most commercial games do these days.
Additionally, it's open-source under the GPL and has an easy to learn but powerful scripting language for building your own scenarios. Due to those two points, the amount of quality mod content is staggering.
I've been playing it since 2004 and it just keeps getting better. I can't recommend it enough!
