The first Natural Selection was a pretty great Half-Life 1 mod of aliens vs. space marines. One team would play as the marines, who would have one player playing from an RTS perspective, buying upgrades and dispensing items, and another would play as the aliens, who would get the ability to evolve into more and more powerful alien forms. I can tell I liked the first game because at some point far back in the mists of time, I paid them money for a pre-order of Natural Selection 2, a standalone, retail sequel which just unlocked on Steam last night. I haven't played it yet (it is still downloading), but the first game was such a blast that I feel I have already gotten my money's worth. And at $25, it's hard to go wrong, in any case.