Way back in 2002, Natural Selection was released as a mod for Half-Life. Now, Natural Selection 2 is being released as a standalone game this Halloween (following suit of the first, which was also released on Halloween). If you're unfamiliar with the Natural Selection game ideology, it was a genre-bending masterpiece mixture of multiplayer real-time strategy and first person shooting. One player per team handles the RTS side, while other players play from the first-person perspective as aliens and humans.
On October 31st, Natrual Selection 2 will be released on Steam and available directly from the developers. If you pre-purchase, your copy will be upgraded to the deluxe edition. Here's the release date trailer: