I use quite a few iPhone applications in connection to my music studio. Here is a list of the ones that I have found helpful.
Awesome Note - by Bridworks
This app is more than just a great to-do list or notepad. I use it to keep track of upcoming projects, marketing ideas, my shopping list, and business expenses. It is really easy to customize for your individual needs and preferences. Color code your folders, look at tasks on the built-in calendar, and customize fonts. Need your information in a different format? Each note can be sent via e-mail or can be printed. In addition, keep all your information backed up by syncing your folders with either Google Docs or Evernote.
The lite version is free, but in my opinion, the $3.99 is worth it for the full app.
For more information, visit http://www.bridworks.com/anote/en/main/index.php
Steinway Metronome - by Steinway & Sons (free)
I have found this metronome to be one of the cleanest and easiest to use. Comes with various time signature settings.
For more information, visit http://itunes.apple.com/us/app/steinway-metronome/id393021343?mt=8
Guitar Tuner - by Alvin Yu (free)
Simple, easy to use guitar tuner. Comes with the option to select different tunings.
For more information, visit http://itunes.apple.com/us/app/guitar-tuner/id310457191?mt=8
Practice Lite - by Sean O'Kelley (free)
Gives you a random scale to play. A great way to mix up what you are practicing. Also, a great way to start off each lesson - have your student play a scale to warm up before starting into the lesson.
For more information, visit http://www.okelleyapps.com/Applications/Applications.html
Question for you: What applications have you found useful for your studio?