And while it might be corny (OK it is definitely corny) I always find the Rocky songs 'Eye of the Tiger' by Survivor and the ROcky theme to be motivating
Anything that has a good beat and you enjoy, I guess. But my favourites are Supermassive Black Hole (Muse), Do Your Thing (Basement Jaxx) and All Right Now (Free). I have a playlist for when I'm at the gym, but I've not had a chance to use it much as I only made it recently, and last time I tried to use it I passed out on a treadmill >.< ...don't let that put you off! Haha...
I love Christian beats! Give me some tobyMac, Skillet, Newsboys, Tye Tribbett, Group1Crew, LeCrae, Jars of Clay, David Crowder Band, etc. But I also love some Will Smith, Shakira, En Vogue, and old school '80s & '90s beats.