Balsamiq Mockups | Balsamiq
I'm just doing a mock up of a new questionnaire builder screen and refreshingly the tool I seem to notice that is used here is Balsamiq. I really am keen on this tool. It keeps the mockups nice and low fidelity which is important to stop crazy people forcing the actual mock up through as the final design on projects.
Anyway for anyone who hasn't tried it, you can use it for free on the website and save you work out as an xml file for later reuse.