Natural Selection 1+2

There is a mod called Natural Selection for Half-Life. The story is simple: MARINES with commander versus ALIENS, that can evolve into other lifeforms. In my opinion, that mod has one of the best gameplay experience. It mixes ego-shooter with strategy elements in a very cool way.
Its so fun, that I still play it since 2001, I think. Somehow I felt like posting about that mod.

Natural Selection 2 is in development, too. The mod team will release that sweet game with their own cool engine retail and with new exciting gameplay ideas. Anyone who is interested should check their site:

http://www.unknownworlds.com/ns2/

There you can follow a really interesting story of making the game, too.
They blog making of concept art, programming tools, sounds and game design issues.