Jog.fm started out as a website. And, now it has an app.
At the website you can type in what your desired running pace per mile is and the site will give you songs have a corresponding BPM rating. In theory when you listen to a slow song while running, you run slower. The same is true when you listen to a fast paced song you probably run faster. Jog.fm will take your desired pace and give you songs that should make you run at that pace when you listen to them.
Another awesome feature is that people can upload their personal playlists. So if you happen to be looking for a half marathon training playlist that has you running at a certain pace, you can probably find it.
The App is very similar in that it will analyze your music library on your iphone and only play certain songs while you are running that correspond with your running pace. It will also track your run using gps and tell you how far/long you ran etc... The app can also be used in the gym or for walking cycling etc...
Just make sure you have a good waterproof case for your iphone if you sweat as much as I do.
The app is $2.99 and available in the Apple App store. As of right now there is no Android version