What app can I use to create mockups for websites?

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/

You can do all sorts of fun things with Omnigraffle as well. And if you're looking for a way to make easy wireframes of current websites, you might do worse than checking out Wirify. Good luck!

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.