Type in how long it takes you to run a mile, and it will give you a list of songs that will keep you on pace (it gives you the right number of beats per minute). awesome. Also has a site for walking and cycling.
The jog.fm iPhone app. "...detects how fast you're running and plays the best songs automatically. You can also easily set a pace for yourself. You can even include a warmup song if you'd like. Need a speed boost? Just press the speed buttons in the app. Cooling down? Press the down arrow. the app plays YOUR music from YOUR iTunes library."