iOS Wire framing/Prototyping

I use three things:

1 - I drew a simple outline of an iPhone and printed out ~200 copies a few years ago. I take a card off the stack and do my first sketches on that sometimes.

2 - Interface Builder works pretty well if you'd like to see what various UI elements look like together.

3 - I use keynote sometimes. I honestly use keynote for everything under the sun...

I've seen a few mockup apps that look kind of interesting, but they always cost more than I'm willing to spend.

We use balsamiq, is quick and easy to draw something up, render it as PDF and send it to a client.
On second hand, we have an designer which makes mockups to if we don't use balsamiq.
And third, when we're live at a client, we use the same technique mentioned above, we have alot of prints of iPad&iPhone frames and we actually draw them by hand.

Those aren't free neither But good luck.

I use iMockups on my ipad.

It actually allows a 'simulated' app. Exporting in various formats are also possible.

Simple, but ideal for business apps.