I have fond memories of playing Natural Selection, arguably one of the finest mods for the original Half-Life. The mix of action and RTS elements, plus the widely different character classes, made for some unique (and often terrifying) multiplayer matches.
Now, 10 years to the day after the mod's 1.0 release, Natural Selection 2 has arrived on Steam—and it's no mere mod. Natural Selection 2 is a standalone game based on its own, proprietary engine, and according to the official debut announcement, it was over five years in the making. The launch trailer suggests the wait was worth it:
Yeah, that definitely looks nice. Early reviews on Metacritic look overwhelmingly positive, as well.
You can purchase Natural Selection 2 right here on Steam. The asking price is $24.99 for the regular version, $39.99 for the Deluxe Edition, and $74.99 for a four-pack, in case you plan to enlist your friends. (The Deluxe Edition includes the original soundtrack, concept art, wallpapers, custom Steam avatars, and an "exclusive in-game marine model.")
Oh, and for what it's worth, the game doesn't sound too taxing on hardware. Steam recommends a 3GHz Core 2 Duo, 4GB of RAM, and at least a Radeon HD 5770 or a GeForce GTS 450.