Ok first of all, I know Natural Selection isn't technically a game, it's a total conversion mod for Half-life. But it's still awesome and completely it's own thing so I am going to talk about it.
Probably the thing I loved most about the original Half-Life was all the incredible mods that came out for it. Most of these were by far more creative and fun than many retail games at the time. The best of them all would have to be Natural Selection.
Natural Selection is a FPS where one team plays as the marines who are basically...the marines from Aliens. They have assault rifles, grenade launchers, heavy MGs and all that good stuff. One marine gets in the commander's chair and plays the game top-down like an RTS. They order their troops around, upgrade their weapons, and build structures. Playing as a marine was my favorite because of how suspenseful it was. This is what every Aliens game ever made should have been like.
The other team plays as the Aliens which are basically...the aliens...from Aliens. Crossed with the Zerg from Starcraft. Yeah, anyway the aliens play completely differently from the marines. There is no commander, and you can pick from a few different species of aliens. One species can run around on walls and pounce, one can fly and one can build structures. Resources are split evenly by the team, and you can use them to upgrade abilities or evolve into more powerful aliens.
The 2 teams fight on maps that are mostly space stations or space ships or colonies. Everything is very dark and industrial looking, and the game has a very cohesive feel, especially for a free mod. Gameplay was pretty well balanced for the most part, and teamwork was vital to winning. The scariest part of the game was when an alien player would get enough resources to evolve into an Onos, which is basically a giant alien rhino. It rips the shit out of everything, and could even swallow marines whole. Yeah, this game was so boss. I can't wait for the upcoming sequel!