Music to workout by...

I am not really someone who listens to a lot of music when I work out, but occasionally I do, and sometimes I NEED it to get through a workout.  If I am running or riding outside I never need music, occasionally when I am on the treadmill at home I will use it, but when I am at the gym I HAVE to have it.  The funny part about it is that I listen to music that is completely foreign to what I would normally listen to, for instance yesterday, I was using my Pandora app and I listened to Beyonce(!), and Ne Yo(!) but  it was Britney Spears "Womanizer" that actually got me going, really?  Britney Spears?  Now don't get me wrong, I will listen to Justin Timberlake pretty much anytime and anywhere but not Britney, it was kind of crazy, but I tell you what, Britney is going to be added to my list because it was a great pace song and I really enjoyed listening to it, of course when I was going to bed I kept singing, Womanizer, womanizer, womanizer, womanizer, you you you ah, you you you ah, a Womanizer, womanizer, womanizer, womanizer...oh great, now I feel like listening to it but I do not feel like working out, this could be a bad sign...

Any other good workout songs out there?  I need a few that are upbeat, fun to listen to that have a good pace :-)