Rated 4 out of 5 stars

Great little mock-up and wire-framing tool! The "libraries" of elements you can add to a drawing are especially helpful.

Using Firefox 11 on Win7. It said Pencil wasn't compatible and wouldn't even list it on the regular Addon search. I installed it from the development site on Google Code that it works fine (http://code.google.com/p/evoluspencil/downloads/list).