Can anyone recommend simple user interface design software?

I seem to remember a startup recently linked on this site that solved my exact problem, but for the life of me I can't find it and Google is no help.

I am looking for simple software (preferably destop) that lets me mock up user interfaces quickly. The software I recall actually had working widgets and would serve nicely for basic 'does it work right' logic.

Am I imagining things or does such software actually exist? If so what is it and why is it so hard to find?