If you're into software development of the GUI kind, then here's a Pencil that you can't do without - and it's free!

If you're a software developer who loves putting GUIs (Graphical User Interfaces) together then this free tool is for you.
Pencil turns your Firefox 3 browser into a sketching tool with just a 400 Kb installation package.

Pencil will always be free and can run on virtually all platforms that Firefox 3 supports.

The Pencil Project's unique mission is to build a free and OpenSource tool for making diagrams and GUI prototyping that everyone can use.

Top features:

• Built-in stencils for diagramming and prototyping
• Multi-page document with background page
• On-screen text editing with rich-text supports
• PNG rasterizing
• Undo/redo supports
• Installing user-defined stencils
• Standard drawing operations: aligning, z-ordering, scaling, rotating...
• Cross-platforms
• Adding external objects
  

Click over to their home page and read the quick instructions for end users to start using Pencil in just half an hour.


Oh, and if you don't use Firefox (for some reason), then there's a stand-alone install (8.6MB) as well WinXP, Vista, Linux.
See here for the download options: download Pencil