I have:
* Ukulele Toolkit, which is a good jack of all trades app: tuner, chord dictionary, metronome, scale finder, etc.
* UkeMaster, which is a chord dictionary, but allows for reverse chord lookup as well, and is easier and cleaner to use than the chord dictionary of the toolkit, above.
* Trainer HD - Ukulele, which is a fretboard learning tool
* UkeChordDetect, which takes songs from your iTunes library and analyzes them for chord progressions
* QInterval, which is a simple circle of fifths.
And
*OnSong, which is a song sheet repository, to organize all your text, word, and pdf song sheets. If they are in chord pro format, it will allow for transposition on the fly, as well as auto scroll, set list composition, and a million other features, including displaying uke chord charts.