What We Like: 
Can't figure out what new song to add to a playlist? Looking for personalized recommendations and "What's Hot?" Well you can now thank Rovi for its AllMusic app which does all that and more! And it's a free app! So go download and start grooving to all your favorite and soon-to-be-favorite tunes!
Why it's cool: 

AllMusic by Rovi takes the best of AllMusic.com and packages it into a convenient app for mobile music discovery. AllMusic lets you tailor your experience, offering personalized recommendations based on music you already like. Read album reviews and artist biographies, listen to song samples and buy on iTunes, get new release updates and share albums with your friends.

API: 

http://developer.rovicorp.com

Application URL: 

http://itunes.apple.com/us/app/allmusic-by-rovi/id424438011?mt=8

Platform: 
iPad
iPhone