Runescape...
But really, you might try Tremulous, it's an old shooter and favorite of mine, but it's not particularly popular, so you'll not find many players on at any given time. It pits Humans vs. Aliens, humans being the usual bipedal gun weilders they are and aliens are fast moving things that look a lot like spiders.
This is what's called a marauder, one of the classes you can play as an alien
And of course the over-compensating human battle suit.
Here's the rundown of the two teams differences.
Humans
- Earn money to buy better weapons/armor
- Are always at a constant max health of 100
- Can wear big armored suits
- Have all the typical shooter weapons: Shotgun, basic machine gun, something resembling the BFG9000, flamethrower, grenades etc. etc.
Aliens
- Collect evoloution ponits by killing enemies to evolve to a better class
- Start at the tiny dretch with 25 hp, to the massive tyrant at 400 hp.
- Some alien classes are weak and fast, others slow and strong with the ability to trample or pounce
At any rate, if you'd like to give it a try, go here:
http://tremulous.net/files/ and downoad the 1.1.0 version for your OS, and yes if you have a mac, it's 100% mac compatible.
hope I helped
-J_l