I'm looking for a free or commercial software for Os X to create mockups for websites. Any suggestion?
Balsamiq Mockups is a good one, fairly cheap and very good - http://balsamiq.com/
WireframeSketcher is a cross-platform rapid wireframing tool.
For mocking up websites I swear by Adobe Fireworks, it might be overkill, but the abilty to have a library of buttons etc and set a master page that carries elements to all the other pages, so headers/footers etc only need to be changed once.
Check out frame box—a "lightweight online tool for creating mockups."
Depending on the definition of "mockup", you might be interested in DENIM.
DENIM is a system that helps web site designers in the early stages of design. DENIM supports sketching input, allows design at different refinement levels, and unifies the levels through zooming.