Actually, it doesn't have to be instrument-specific.
I have exactly 1 hour each day to play guitar (sometimes less). So I have to be very structured and intentional with my practice time if I want to make any progress at all.
Basically, I'm looking for an app that does what I'm currently struggling to do by hand with spreadsheets, timers, etc: organize my practice, set timers & metronome for each exercise, track progress, etc. I'd love to have an app that I can put all this stuff into, and it just progresses from one exercise to the next, with rest breaks in-between, without me having to take my hands off the guitar... manually set timers, click a mouse, etc.
I've found a couple apps on iTunes, but was hoping someone here had some personal experience and could recommend something.
Update: I've begun skipping lunch. As long as I can survive on only 2 meals a day, I gain almost an extra hour to practice. Problem solved!