As you might already know, I am a fan of all things Apple, Mac, and iPhone related.  I own an iMac G5, MacBook, Airport Extreme, Apple TV, iPod Shuffle, iPod Classic 160gb, and iPhone 3G.  Since Apple opened the App Store to developers, it has been flooded with good (and not so good) music apps.  Here are the music apps I think every musician will find useful.  All of the icons will link directly to the download page in the App Store in iTunes.  

Cleartune by bitcount ($3.99)

Cleartune (by bitcount) is my goto tuner.  Period.  I own an expensive Korg needle tuner and tested it against the cleartune iPhone app and gotten the same results.  The developers were smart enough to build in an auto-rotate feature, so you can turn the phone putting the mic on the top instead of the bottom.  It also includes a very loud pitch pipe spanning over 9 octaves.  I use that feature to practice and tune long tone scales with a drone, as suggested by my friend, JoetheMusician.  It does require a mic, so if you own an iPod touch you will need an external mic.  I highly recommend this app for every musician who owns an iPhone or iPod touch.