Review our UI mockup tool, ScreenSketcher
dpcan: I purchased this program way back when you did the promotion post to the Business of Software forum and I've been using it regularly ever since.  It's so much more responsive than Balsamiq and I absolutely LOVE it.EDIT: I just read your blog post, and it looks like you took over this product from the original creator of whom I purchased it from.  I hope you allow us to upgrade if you'll be maintaining it better.