I found this game on the internet:
It's called tremulous. It's cross-platform, and online. So if you have slow internet, don't get it.
It's also free!

description:
do you like First Person Shooters? Do you like Real Time Strategies? Well this is the game for you!
In this game, you are given a choice of 2 classes: A builder class, and a Fighter; both upgradable. You play as the one you choose.
There is 2 races: Aliens and Humans.
Humans:
Bases take long to build, but have automated defense systems.
Aliens:
Bases take less time to build, and have an upgrade that let's you add poison to your bite.

You guys should really try it! (especially you, StrafeHunter! It's kinda like TF2, only free!)