Plus, this games also has elements of a first person shooter game, real time strategy and additional multiplayer online game features. It's based on the Quake engine, and has two main skillsets of aliens and humans.
The game I'm talking about here is called: Tremulous.
It's free, open source (yes, that means hackable too...) fast and easy to learn. The objective of the game is this: There are two teams, humans and aliens. Each team has to build their own base and defend it, while attacking the enemy base to destroy it. Plus it looks pretty good too.
Naturally there are several unique elements in the game. For example, you need to be able to shoot (with several different kinds of weapons that work differently), build a base (both human and alien bases are different) and learn how to use an alien (because aliens can't shoot.)
Plus, since its free there are loads of people all over the world playing the game. You too can host your own game server from your very own computer (provided you have a good internet connection and a fairly fast computer) and all you need to play is a regular mouse and keyboard.
We'll instead of me telling you more about the game, why not give the game a try. May be you'll like it, may be you wont. But it's free.. so you've got nothing to loose.
Got a question, tip or comment? Send them to firstname.lastname@example.org and we'll try to answer it in a blog post!