I was thinking about what to post and I’ve got nice twits like this one:

@sbohlen I am quite impressed by this http://bit.ly/ci3fxn Its no #balsamiq but its still amazing (and free); runs in and OUT of your browser

So why not to try that thing. So I installed Pencil add-on.

Here is screenshot of my try:

Toolbar on the left is rich of different shape collections. Sadly I did not find one for UML diagrams, but they could be easily added and that is great.

So this is the simplest way to have light prototyping machine just in your browser.