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.
Tempo by Frozen Ape ($1.99)
It took me a while to find a metronome program that has all the features I wanted, and Tempo has got them all. A loud click, landscape mode to expose the speaker on the bottom, both a click and a light indicating tempo, tap a tempo, and easy to change tempo were the features I was looking for. I tried several free and paid programs and this one had them all. It has support for playlists, various meters, and several subdivisions. To quickly jump to the next tempo that is a multiple of ten, simply hold the up or down button. Hands down this feature rich metronome kicks the pants off of every other metronome app in the store and even most $30 physical metronomes you'll find at your local music store.
iReal Book is a must have for any jazz musician. It contains the chord changes for about 900 included songs, and I expect that number to continue to grow. I've been very impressed how often the developer updates the app to enhance functionality and to add new songs. On top of the included 900 songs, you can easily add more by using an included free song editor, or by downloading user-generated songs from the forums. It has transpositions for horn players, as well as an easy way to transpose any song to any key on the fly. Tunes are organized alphabetically, by composer, or by style. You can create a playlist for a gig and there's even a night mode for low light situations. I've used the program several times in gig situations and am very happy with it. I'm hoping that developers see the potential and will be making great music reading applications for the iPad, utilizing the screen real estate.
Both of these internet radio apps are a great way to stream music and to find new music and artists. You can tailor stations to your preference, building stations by similar artists or genre. You can also like/love tracks, bookmarking them for later purchase. Banning songs will take them out of rotation of that station for at least a month. Pandora uses the Music Genome Project to help you discover new music, while last.fm can actually scan your music library and recommend new music for you. I think both services are great and to top it off, it's all free (well ad supported, but the artists need to get paid too).
If you really don't want to spend the $1.99, Metronome - reloaded is a decent free alternative. It's very simple, with a nice loud click. Just like a piano metronome, you can use the slider to select tempo. I also like the inclusion of tempo markings in the upper left, in case you're not sure how fast 'comodo' is. Though the slider does work, it can be hard to get to the exact tempo you want. Thankfully, there is an easy built in solution. Just touch the number at the top to bring up a keypad to enter the tempo you want.