Here’s a list and comparison of Mockup and Wireframing tools: http://socialcompare.com/en/comparison/mockup-wireframing-design-tools
After a brief look at the list, I decided to give MockupBuilder a try.
I found it very intuitive to use and it provided a ton of mockup capability and widgets. I used balsamiq briefly in the past, and I think this one is better. In about 4 hrs, I was able to mockup in pretty high detail an entire website.
It’s also free (for now).
The only downside was that I needed to use Safari. I wasn’t able to get it to work with Chrome or Firefox on my Mac. I was able to do some drag-and-drop but couldn’t type for some reason.
I think it’s a bit lacking in terms of collaboration and noting comments but I haven’t really explored this a lot.