Tremulous

From the Developers
Tremulous is a free, open source game that blends a team based FPS with elements of an RTS. Players can choose from 2 unique races, aliens and humans. Players on both teams are able to build working structures in-game like an RTS. These structures provide many functions, the most important being spawning. The designated builders must ensure there are spawn structures or other players will not be able to rejoin the game after death. Other structures provide automated base defense (to some degree), healing functions and much more...

Player advancement is different depending on which team you are on. As a human, players are rewarded with credits for each alien kill. These credits may be used to purchase new weapons and upgrades from the "Armoury". The alien team advances quite differently. Upon killing a human foe, the alien is able to evolve into a new class. The more kills gained the more powerful the classes available.

The overall objective behind Tremulous is to eliminate the opposing team. This is achieved by not only killing the opposing players but also removing their ability to respawn by destroying their spawn structures.

My Comments
Imagine Team Fortress Classic meeting Alien VS. Predator and you've got an inkling as to what this game is like. The Aliens have a few cool moves like walking on walls, but the way you'll have to move run about as super-fast speeds gets a bit disorienting, and having mostly melee attacks takes some of the fun out of the game. If it's an FPS, I want to shoot things, dammit!

The best thing about Tremulous is that (at least in Australia) there are plenty of people online to play against most of the time. Though I do wish they'd all switch to playing Nexuiz.

Oh one more thing, the Mac version of the game isn't official like the Linux and Windows releases, but I've yet to experience any bugs with it so you should be OK.

Aliens don't scare me, give me the download!