Samplr is my favorite new sound mangler. Favorite old one is Filtatron.
Just had a fun afternoon playing those hooked into my focusrite and my netbook, but just live playing along side some beats I did in Ableton with Poise and some good samples.
Soon with Beatmaker 2 and Audiobus plus more apps jumping on board for Audiobus, I'll have these fun jam sessions just with the ipad running straight into the powered speakers and maybe one controller hooked up.
Mobile apps as sound modules are really cool as it is though.