[slindstr]

I was actually in a meeting today where someone was showing us mockups using Balsamiq. First time I've ever seen it - looked pretty cool. He was able to make interactive PDFs so when you click on an element of the mockup (say a button) it showed the action (like popping up a dialog). He seemed to like it well enough too, but like you said it's not free and requires Adobe Air...

[rabidonrails]

I've been using Balsamiq for a while now and I love it. Sure it runs on Air and isn't free, but it's one of the best purchases we've made as a bootstrapped startup. Also, the group that runs Balsamiq is awesome and the CEO -Peldi?- writes a great blog.