I do GUI prototypes from time to time and as it usually happens with programmers doing design these things start getting some code too early, even before the concept is done. Later on the code is injected in the project but is removed after several iterations - it no longer fits the design.

Yet another prototype and yet another code-first condition, but this time I can see GUI before a line of code is written.

Pencil

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

This is really easy - The basic Stencil set is composed of Gtk+-like widgets and WindowsXP ones. The googlecode web site contains more widgets, like ExtJS GUI one.

The example below shows a prototype of Windows XP Log On screen with one usability improvement and some annotations.

Visit the web-site and get your free Pencil. Have fun doing prototypes!