I use penultimate to pretty much replace my note books to sketch out new ideas.
Task paper to plan projects.
Numbers for various little spread sheets I still have to keep, e.g. tax, bills.
Tab toolkit for well, tabs.
One day I will get better at guitar again and record something on Garage band. Until then I use Garage band and the Apogee jam to replace all the effects boxes I could never justify buying.
I've started drawing again thanks to Brushes.
Occasionally Textastic to review code on the go. I can fix small things if I have to or write up pseudo code that I fix up when I get to my main machine.
Some of this is not technically producing I suppose, but my iPad is now a very convenient part of my process.
And while this doesn't quite count as producing, a couple weeks ago I pulled up sheet music in Safari, put my iPad on a piano's book stand, and started playing a song that a friend had mentioned only minutes before. You'd never do this with a traditional computer, but today it doesn't even seem that odd or amazing. Sure, it was "consumption": I was reading sheet music from the iPad. But if that's not intellectually stimulating and productive, I'm not quite sure what is.