[edit: and by "hip hop" I mean gangsta rap, not stuff like Digable Planets.]
Mogwai, Pelican, Explosions in the Sky, Pinback, Castor, Elliott, National Skyline, Isis, My Bloody Valentine, Jesu, Portishead, The Mars Volta, Low Frequency in Stereo...
Lately, Japancakes' Loveless cover album has been in non-stop rotation. Good stuff.
I tend to listen to some of the electronic / trace channels in iTunes radio: ETN.fm, SENSE.FM, XTC Radio, AH.FM, DI.fm, Philosomatika ... it really doesn't matter what the actual music is as long as I like it and it's not distracting.
I do this too, often using the same track for several days in a row. When I do this without headphones my wife complains for some reason ;-)
Like what snoopy does in this video:
instrumental stuff usually works best, but i like variety.
i've recently become very fond of danger and m83.
The only song you ever need. Endless loop.
But usually it's just post rock or techno or something else that's kind of boring and instrumental.
I listen to a lot of different types of music. In order of popularity, probably reggae, folk rock, bluegrass, punk and then various other miscellaneous odds and ends. I tend to prefer songs with lyrics that are either clever or witty, or both. A particular favourite of mine at the moment is "The Battle" by Lady Sovereign et al. Lyrics like "Eating MCs like Ready Brek" and "I kill you little kids like morning after pill" never fail to make me smile.
Also, allow me to apologise in advance for the trauma caused by placing the image of a middle class white British guy singing reggae in your mind.
If I want to get focused quickly, I listen to some particularly horrible electronica (Winnipeg Is A Frozen Shithole by Venetian Snares is a good example). If it's three in the morning, I listen to some minimal electronica (Vladislav Delay, B. Fleischmann both good).
Otherwise, it's whatever I'm enjoying at the moment (recently: brutal hardcore like Circle Takes The Square, weird indie like Frog Eyes and Sunset Rubdown).
Finally, long albums are good because of the longer gaps between ends of songs and ends of records.
Talking Heads is really great hacking music. "Born Under Punches" is a really good example of a song that you can hack to.
I listen to an insane variety of music while hacking. Some highlights of the past month or so: Bob Dylan, Ruby Suns, Evangelicals, Paul Simon, Anton Webern, The Dodos, Xiu Xiu, The Velvet Teen. A pretty good variety of classic rock, indie rock, experimental and classical.
Armin Van Buren - A state of trance
Above and Beyond - trance around the world
Tiesto - club life
and plenty more.
I love electronic music.
I find trance great, because it typically has no lyrics, or even if it does then they are very simple so they just become sounds that are part of song itself.
Once in a while though when I'm at some crossroads and need to truly concentrate on something for 3 minutes I just mute everything around me for total silence.
If I'm in real trouble and really need to focus, I break out Philip Glass. His early stuff, like Music in Twelve Parts, is mesmerizing, really helps concentrate.
My answer is pretty much the same as before: Anything my brain doesn't try to process. I'm still listening to Nujabes - Modal Soul, actually.
Lots of experimental electronic music and glitch. Seems perfect for the kind of programming I do...