On TUAW we've looked at many apps which let you slow down the tempo or change the pitch of a song to help you practice music more effectively. And that's exactly what Anytune does, but with a great degree of detail and some extra features to help even further.
Anytune is a free, universal app, but with in app purchases (IAPs) to add extra features and functionality, turning Anytune into Anytune Pro+. The free version of Anytune lets you import music from your iOS library, adjust the pitch and tempo with great detail -- without affecting sound quality -- and select and mark certain parts of the song to loop for focused practicing.
IAPs (from US$1.99 to $12.99) include features like higher-quality audio, importing music from Dropbox / email / WiFi (although I don't know why you wouldn't just import songs from iTunes to your iDevice, but perhaps there are some niche users who will take advantage) and AirPlay support.
Some standout IAP features include "Step-It-Up" interval trainer, to challenge you in your practicing, "FineTouch" EQ lets you draw out particular dynamics of a song (guitar, vocal, bass) and exporting an adjusted song for practice at a later stage.
If you're looking for an app to help you practice your instrument to music, Anytune is a well-designed and easy-to-use app. It's certainly worth checking out, especially since its core features are free.
Anytune is available from the iOS App Store now.