Huh? No, you're thinking of the iPhone. OSX is pretty much open, though nothing like linux's creamy core. Valid mistake I suppose though, as the iPhone and related things are under an iron fist. We Mac users just tend to not use the crap that does exist.

My currently-playing music is "Kumikyoku Nico Nico Douga (Chorus Grand Finale ver.)" ripped from a collab video from (can you guess?) Niconico Video. It in no way fits CC. At all. Honestly, it's painful.
Other than random stuff like that, it's 18 gigs of anime and video game themes, with sprinklings of assorted extras. I strongly avoid english lyrics, but really great songs are exceptions.
CC goes well with some of Halo 1's themes (as does life in general), and Guilty Gear's as well.

I usually hook up my MP3 to my speakers.

I play I Will by radiohead to start off and work my way up the tempo scale to Electioneering. Then I blast some Muse, and move up to Insomnium.

Gotta get me some of those. How about Soldier's Poem? Sometimes when I feel the need to be dramatic I'll play MCR's The Ghost of You and Coldplay's X&Y. Somehow those two songs bring up visions of world war 2 soldier's nightmares. For futuristic stuff I'll play Muse's Sing for Absolution, Radiohead's Go to Sleep (Little Man Being Erased), or some GITS soundtracks.

I just mute the music in-game, then go to playlist.com, I have disturbed, metallica, system of a down, avenged sevenfold, and a lot of misc stuff. The only problem with my method is the lag it causes.

I usually have System of a Down, Tool, Alice in Chains, Stone Sour, and Disturbed playing while I play CC. So basically killing music.

Either "Battle Without Honor Or Humanity" or "Ride of The Valkyries", depending on the mood I'm in.