Pencil is a pretty nice open-source GUI prototyping tool. The only little problem for Debian user is, Pencil depends on Firefox and not on Iceweasel. If you want to bypass that problem very easy, you can download pencil as Firefox Extension. But if you want to use Pencil as native application you have to download the deb package and will see, that it depends on Firefox. Iceweasel and Firefox are the same things, but have different names.
Here is a Wikipedia articel about Mozilla Corporation software rebranded by the Debian project.You can run dpkg also with --ignore-depends=firefox, but that isn't a very good solution.
Ravi Teja wrote a good post about Modify dependencies, add/remove files of debian(.deb) package. Now we just have to change firefox to iceweasel in DEBIAN/control of the package.
dpkg-deb -x evoluspencil_2.0.5_all.deb dir_tmp dpkg-deb --control evoluspencil_2.0.5_all.deb dir_tmp/DEBIAN
Package: evoluspencil Version: 2.0.5 Architecture: all Maintainer: Nguyen Hong Thai
Installed-Size: 8952 Depends: iceweasel Section: unknown Priority: extra Homepage: http://pencil-project.org/ Description: Evolus Pencil Pencil is an open source GUI prototyping and sketching tools released under GPL.
dpkg -b dir_tmp evoluspencil_2.0.5_all.deb
sudo dpkg -i evoluspencil_2.0.5_all.deb