Some people can work out without music. Some people even enjoy working out without music. Something about focus and concentration and all that.
I am not one of those people.
I am a weakling who will walk right back out of the gym if I realize I'd forgotten my iPod. I need the crutch of the oontz-oontz-oontz to even get me going.
Here's the semi-embarrassing playlist you'll find filed under "treadmill" on my iPod ("stair mill," "bike," and "elliptical" get their own, thank you very much).
Here's a great site for finding new workout music (RunHundred.com). There, too, you'll find this YouTube video showing a demo of an app that allows you to determine the beats per minute (BPM) of a given song. So if you're looking to assemble high-BPM songs for a run, this could be a great guide:
If you've got a kick-butt playlist of your own, by all means, please share!