We've been using plain paper & pen to create mock-ups, but it's kind of hard to collaborate online (many interactions of scan-email-comment). So far I liked th

We've been using plain paper & pen to create mock-ups, but it's kind of hard to collaborate online (many interactions of scan-email-comment). So far I liked this one: http://www.mockupscreens.com http://www.mockupscreens.com . There's another one called Axure, but it's quite expensive: http://www.axure.com http://www.axure.com . I tried Denim a while back (http://dub.washington.edu/denim/ http://dub.washington.edu/denim/ ), but it's a pain to use with a mouse (Denim was designed for tablet-pcs). Drawing tools are ok, but they do not have ready-to-use UI components like buttons, drop-downs, etc... Visio has UI components, but it's expensive. Are you using any tools for web app mock-ups?