- RTS
Starcraft 2
Don't know of any others on the Mac since I dislike this genre.
- FPS
Call of Duty 4
Team Fortress 2
Those are my 2 favorites by far, but there's also...
Bioshock
Killzone
Left for Dead 1 and 2 (coming soon to Mac)
Half Life 2 (and its expansions)
Doom 3
Halo
And some smaller ones...
Warsow (free)
Quake Live (free, browser-based)
- MMORPG
Warhammer Online (has an endless trial, that allows you to play forever, but you can't level beyond 10)
Dofus (restricted free play)
EVE Online
Fallen Earth (looks a lot like Fallout 3)
City of Heroes
And of course the juggernaut: WOW. Decided to try it recently. Been playing for only a month, so although I've seen little of the game and experienced none of the most epic and vital activities within (raiding, world pvp, arena), I'm already thinking it might be the funnest game I've played already.
- RTS+FPS+MMORPG (seriously, there's a game that combines all 3 you asked about--and it's free)
Savage 2 (3rd person melee combat, first person ranged combat, persistent MMO elements and a RTS role for one player on each team)