Pencil is a Firefox plugin that allows you to draw diagram and prototype an applications Graphical User Interface (GUI). Pencil is similar to Eclipse WireframeSketcher. You can install the latest version of Pencil from the Mozilla addons site. Once the Firefox extension has been installed you can launch it by selecting Tools | Pencil Sketching from the Firefox menu. The Pencil plugin will open in a new Firefox window for you to draw you GUI. In the left hand side of the Pencil canvas, you will have a set of palettes of different shape collections, such as GTK Widgets, Windows XP Widgets, annotation, and common shapes. Just drag, drop, and draw the different widgets from the palette to the canvas to draw a UI prototype.

With Pencil, you can create rapid screen mockups and UI prototypes, add your own custom shapes to new palettes, save your work as a Pencil document format, export your diagram or UI as a PNG file,

I did have issues running the current version of Pencil on Mac OS X, but the Pencil project is Open Source software released under the GPLv2.