What an awesome idea for a thread. Everyone's posted some awesome songs, many of which are already on my playlists for the gym. I personally go for a huge mash-up of stuff. Anything that's driving, has a good beat, or can get you pumped... I just slap it all together and sweat baby sweat.
Still a huge fan of all the music I grew up with, y'know.. the 90's rock, R&B, etc. But I never rule out anything interesting.
Eminem - Lose Yourself
Fatboy Slim - Weapon of Choice
Finger Eleven - Paralyzer
The Fratellis - Flathead / Henrietta
The Futureheads - Struck Dumb (One of my fav. treadmill songs!)
Jimmy Eat World - The entire "Chase this Light" CD
Jurassic 5 - One Two / The Game / Freedom (Great beats)
The Kills - Cheap and Cheerful
M.I.A. - Paper Planes / Galang
Madonna - The entire "Confessions on a Dance Floor" CD
3OH!3
Black Eyed Peas
Lady Gaga
Daft Punk
Shinedown
Seether
The Descendents
Fall Out Boy
So many other things as well... but I won't ramble on for forever.