Best (free/open source) mock up tools?

mockup-toolopen-sourceprototype

I am looking for a very good (easy to use, good drawings) mockup tool that allows u to draw up page prototype pages.

I know of pencil, but I use an x64 firefox which I ve not found a pencil plugin for (Any one with pointers to where I can get one?)

I ve also heard of MockingBird which is available online (pls correct me if I am wrong). Outside these two, are there other easy to use, and free ones?

Regards.

Best Solution

JustInMind has a free (limited) version.

In terms of tools that are Open Source, Pencil is the only one I've found. Have you tried the latest version of Pencil? When I downloaded it, there was a package installer (as opposed to installing it as a plugin for Firefox directly). Perhaps it's more standalone now and will work for you?