evoluspencil – The Sketching and GUI Prototyping addon for Firefox
The Pencil Project's unique mission is to build a free and opensource tool for making diagrams and GUI prototyping that everyone can use.
Pencil will always be free as it is released under the GPL version 2 and is available for virtually all platforms that Firefox 3 can run.
Install evoluspencil in ubuntu
download .deb package from here once you have .deb package install by doubleclicking on it or run the following command from your terminal
sudo dpkg -i evolus-pencil_1.2-1_i386.deb