iReal Pro offers an easy-to-use tool to help musicians of all levels master their art. It also simulates a real-sounding band that can accompany you as you practice. The app also lets you create and collect chord charts of your favorite songs for reference.
It's a book -- Create, edit, print, share and collect chord charts of your favorite songs for reference while practicing or performing.
It's a band -- Practice with a realistic sounding piano (or guitar), bass and drum accompaniments for any downloaded or user-created chord chart.
- [Added] Ukulele chord diagrams and font
- OS X 10.7 or later