From Groundcontrol Aps:
ProChords for iPhone is a unique iPhone application developed to write songs with professional chord progressions, in an easy and intuitive way. In the process of writing a song, it offers an extensive variety of options as to what chord could be played next, after the sequence of chords you have already chosen during a writing process of a new song. These suggestions are based on thousands of successful songs and may inspire you to find and choose new chords, which you would not normally have thought of in a particular context. ProChords for iPhone is very useful for songwriters looking to compose songs of professional standards, without having to spend years studying music theory. But it is equally helpful for professionals who are looking for new paths and alternative ideas.