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Natural Selection is a team based mod for Half-Life 1, with a “aliens versus humans” play style.

you can view the Natural Selection web site here:
http://www.unknownworlds.com/ns/

you can download the 3.0 client here:
http://www.unknownworlds.com/ns/view?action=files

you can view the game’s trailer here:
http://www.unknownworlds.com/ns/view?action=trailer