I'm certainly no expert in UI design, but it's nice to mock-up things before you start committing them to code.
I like what I've seen from Balsamiq (http://balsamiq.com), but $79 for a license seemed a bit of a stretch for my modest needs.
I found the free app "Pencil" (http://pencil.evolus.vn) seems to work pretty good.
You can use Pencil as a Firefox plugin, but I ran it as a stand-alone XULRunner app.
Documentation is kind of sparse, but here's one useful link :
Exporting from Pencil is kind of flaky, but the above link gives some hints. I found I could export as HTML, and then print the HTML page as a PDF (attached).
I'm not a UI guy, and don't plan to live in Pencil, so it works for me. Any other options out there?
Here is a good list of UI mock-up tools :