What UI prototyping software have you found most helpful?

Answer 1

The best tool I have ever used, and the one that I have used every time since, for every project that requires UI design, isn't software at all - the paper prototyping method.

http://www.amazon.com/Paper-Prototyping-Interfaces-Interactive-Technologies/dp/1558608702/sr=8-1/qid=1170696716/ref=pd_bbs_sr_1/103-9575422-4293450?ie=UTF8&s=books

On the surface it is very simple, but it can become a very powerful and sophisticated process. In my experience, nothing works better.

Some of my colleagues who like to use software instead recommend qt (http://www.trolltech.com/products/qt)