Tuning, drum machine, metronome, backing tracks and a chord and scale finder.

Tuning
The best tuning app I've found so far is DaTuner.
The best tuning app I've found so far is DaTuner.

Drum machine
Electrum drum machine - useful for quickly setting up beats to play along with.
Electrum drum machine - useful for quickly setting up beats to play along with.

Metronome
Mobile metronome - simple, easy to use.
Mobile metronome - simple, easy to use.

Blues Backing Tracks
GC Battle of the Blues -An app which gives you fast access to the Guitar Center US's blues competition backing tracks.
GC Battle of the Blues -An app which gives you fast access to the Guitar Center US's blues competition backing tracks.

Chords and scales
JamBox - displays chords and scales quickly and easily.
JamBox - displays chords and scales quickly and easily.