If you have an iPhone or iPad the PianoTrax app will change key for you.
There is also a software program called "Amazing Slow Downer". It will allow you to modify the Mp3s you have downloaded on your computer. You can adjust tempo and key independently of each other. You can save the modified Mp3 on to your hard drive.
Amazing Slow Downer is also an app for your mobile device. You can try ASD lite for free, but you'll only be able to listen to the first 1/4 of your song. I have found that saving your songs to Dropbox on your mobile device is the easiest way for ASD to access your songs and make adjustments. You can also export your song in the new key and/or tempo.