Try looking at the following collections of applications:
http://speckyboy.com/2010/01/11/10-completely-free-wireframe-and-mockup-applications/ or http://mashable.com/2012/06/07/mockup-tools/
Question
Does someone knows a tool to help create a mockup from an existing site easily ?
Solution
Try looking at the following collections of applications:
http://speckyboy.com/2010/01/11/10-completely-free-wireframe-and-mockup-applications/ or http://mashable.com/2012/06/07/mockup-tools/
OTHER TIPS
MockGen can generate Balsamiq and ForeUI mockups from existing web pages.
Insanely customizable output.
http://www.mockgen.com/desktop (I'm the developer)