I am working on a few ideas for iPhone apps. Although there are some good apps and software out there for initial prototyping I find some key pieces of hardware useful.
I use a small pocket spiral notebook by Mead and a box of 8 erasable color pencils from Crayola to do my first wire frames. From there I may move on to other software, but for the time being this allows me with some flexibility and quick access to other sketches that might give me some inspiration.
What do you use?