Here’s a few mobile (specifically Android) apps that I use:
Cleartune – A really good tuning application. When I compare to to my electronic tuner and it seems to be pretty accurate.
Metronome Beats – I haven’t used it much yet but it’s a metronome app that has different sounds rather than the computer (heart monitor-like) sound that most computerized metronomes have.
MusicTutor SightRead – It was an app recommended by someone else on this site. It helps you learn to read and write music.