I have been playing this mod for either Halflife or Counter-Strike depending on what media you have. It's a great mod for anyone who enjoys the Aliens vx. Man type of genre. Plus it's not just shoot em up, there is a whole overhead commander part so it add's a piece of a RTS to the mod. It's about 100mb to download and it is worth a try.
http://www.natural-selection.org/
http://www.natural-selection.org/images/m-marine2.jpg
