pencil prototyping tool is a cool xul application , runs on ubuntu , windows ...

I found the project now
If you need to create quick prototyped web pages or some app forms

on ubuntu natty 64 (standalone - seems that extension is not supported yet in firefox 4.0)
sudo apt-get install xulrunner-1.9.2

wget http://evoluspencil.googlecode.com/files/Pencil-1.2-0-linux-gtk.tar.gz
tar -zxvf Pencil-1.2-0-linux-gtk.tar.gz
cd pencil

xulrunner application.ini