This one of my favourite apps. While I’m working on learning and hearing jazz progressions, iRealbook via its forums let me store chord changes for 1000s of tunes from jazz standards to The Beatles. There are no melodies for copyright reasons. One of the best features is that I can loop sections of tunes to work on bass lines or improvisation. On the gig the best feature is that I can transpose a tune to any key. I can also make playlists for anyone I work with so that I am prepared for the next gig.
iGigBook lets me store all my fakebook PDFs on my iPad. A fakebook contains a transcription of the melody and chord changes for a tune. Currently I have about a dozen books on my iPad and through the app I can search for tunes in a matter of moments and be ready to go on a gig. I can also prepare set lists for future gigs.