Update: The original tool seems to be no longer in development (last release from 2013-09). @wb9688 mentioned that there’s a fork: https://github.com/prikhi/pencil
Update 2: @nradk commented that the fork is no longer maintained. https://github.com/evolus/pencil is another fork, "in active development, and it is actually a rewrite of the old one".
Pencil (License: GPLv2) is a "GUI prototyping tool" which is available as
- a stand-alone tool for GNU/Linux (*), Microsoft Windows, and Mac OS X, and
- a Firefox add-on (**).
(*) It’s included in the software repositories of Arch Linux and Fedora (as pencil
).
(**) The Firefox add-on hosted by Mozilla seems to be out of date (currently it’s version 1.0.6 from 2009), while the version hosted on Pencil’s download page seems to be the current one (currently version 2.0.5 from 2013).
I use Pencil as Firefox add-on. It opens a new window (with no Firefox chrome, it got its own menu bar etc). Sometimes, it gets kind of slow (e.g., when I rearrange graphics), but here my old PC might be to blame.
The graphics are called "Shape Collections" or "stencils". It ships with a few sets for different interfaces (including Web). Additional graphics can be imported. Here’s a sample wireframe I made to get an idea of some of the included graphics:

Export of wireframes is possible as PNG, HTML, PDF, SVG, and ODT. A nice thing about the HTML ("Single web page") export is that you can add hyperlinks that link to other mock-ups (included in the same Pencil document), i.e., you can simulate user interactions.