I can't stop playing with Loopy HD. If you have not experienced the joy of creating your own loops then you need to get your hands on this app and start looping. This app was brought to my attention when a colleague showed me a clip of Jimmy Fallon and Billy Joel singing multi-layered doo-wop parts into this app on the Tonight Show. The app has a unique interface that is both visually appealing and oddly intuitive.
Another app that I often find indispensable is called Anytune Pro+. This app is somewhat pricey in comparison to others (free ones in particular). This app has the ability to slow down any music without changing the pitch. For anyone who has tried to transcribe music you will understand how important this is. I spent a good deal of time transcribing guitar work that moves faster than my ear will allow me to efficiently reproduce. With this app you can bring it to a crawl and not affect the pitch at all. Amazing.
I will try and keep you updated on the apps I enjoy. If you share some of your favorites with me, perhaps I can enjoy those as well.