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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...

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.

I enjoyed using mockingbird. Can't collab in free, but can show other people and have them look through it. Balsamiq got old with popups asking me to upgrade.

You should check MockupTiger too, it is also using html5 so works only in chrome and firefox. But you can host it on your own domain or use it on desktop.