What's the best Wireframing tool?

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.

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.

Try Balsamiq. It uses Adobe Air and runs properly on Ubuntu.