After winning the Extend Firefox 3 contest, I thought it might be worth checking out Pencil to see what all the buzz was about.  This add-on for Firefox is an open source tool for creating diagrams and GUI prototyping that everybody can use.  As they put it, “With the power of the underlying Mozilla Gecko engine, Pencil turns your excellent Firefox 3 browser into a sketching tool with just a 400-kilobyte installation package”.

Some of the features included are:

Built-in stencils for [...]