What's the best Wireframing tool? [closed]

I'm looking for something similar to iPlotz or Mockup.

I've found the Pencil Project, but it requires xulrunner-1.9 (which seems to be incompatible with xulrunner-1.9.2) in order to run as a standalone application. It can be used as a firefox plugin... but it is a bit slower.

The error on my desktop (Ubuntu 10.04) is: Could not find compatible GRE between version 1.9.1 and 1.9.2.*

Does anyone know other software?

Edit: Open-source software is preferred, and it doesn't matter whether or not it's free.

Solution 1:

WireframeSketcher is a rapid wireframing tool for Eclipse-based IDEs like Aptana, Zend Studio and the like. It also comes as a standalone version for all major platforms including Ubuntu. Give it a try.

Solution 2:

After some research, I found Dia, and it's exactly what I was looking for: simple, open and stable.

Maybe I explained my needs badly. More than just software to design layouts, I also needed something that provided the ability to build flowcharts. I tried ArgoUML, too. It looks great, just a bit complicated.