I've been using Guitar Pro 6 to make accompaniment tracks from Noad's "Solo Guitar Playing", but I think there are a lot of other things I'll be able to do with this program. For one, there is a website, http://www.classclef.com/
that hosts a bunch of classical guitar scores in the .GPX file format (as well as PDF and MIDI). Creating your own sheet music is also ridiculously simple, and allows newbies like myself to "play back" what I've entered to make sure that I'm scoring it correctly (fantastic learning tool in this regard). Also, trying to learn unfamiliar music by sheet is difficult for beginners. With this program I can enter in complicated sections and hear how they're played. I'll use this to practice counting out the beats without tripping myself up. This is helping out considerably with my ability to read the timing of notes.
For $40 bucks, not a bad deal.
For useful freeware, try mandolin tool (http://www.tomkysoft.com/
), which will give you "chord and scale charts for guitar, banjo, ukulele and mandolin, a tuner, a metronome with tab tempo and a music player which plays music files at slower speeds without altering the pitch."
*** it now occurs to me you mean specifically iphone/ipad/android applications. Hope you don't mind my recommendation of PC applications. I don't have an ipad/iphone/android device).***