There are several apps that try to be a mobile song sketch pad, but I think Okee Doke have finally nailed it with VCVCBC for iPad. This app has a lot of clever design, in both functionality and appearance. You can figure out chord progressions with the on-screen, scale selectable, guitar as you hum along to the lyrics you've plugged in. It even has a built in rhyming dictionary to help with the lyric writing process, and some other useful features for working out melody and rhythm. It is also now on sale for $5!
Okee Doke has put together a series of tutorials, and participated in a 30 Songs in 30 Days challenge. They finished the challenge quite strong, with an amusing music video. The tutorials are really well done, Ben has a natural personality for these!