My favorite is an iPhone app titled FourTrack. It is, essentially, a 4 track recorder and it has become a catalog of recorded musical ideas. With audio file import/export features, a metronome and extensive EQ capabilities it turns the phone into a miniature practice amp and recording studio.
Another iPhone app I find quite useful is titled Reverse. It displays a fretboard and identifies chords based on your selection of notes (tap a string in a fret).
Planet Waves has a do-it-all, multifunction app titled Guitar Tools, which includes their approach to a reverse chord lookup, along with a massive visual chord chart, a very useful visual scales chart, a metronome, a tuner, and more.
I have Peterson's tuner app title iStroboSoft, but I typically rely on a tuning fork (force of habit).
It still amazes me all this stuff fits in a phone, which fits in your pocket.