This is something that has been around for a while now. I first tried it years ago but I had not any real usage for it back then. Last week, I needed something to capture a UI change I was planning to do in my project. My first though was just draw on a piece of paper, but I have to e-mail this to someone off site.
What should I do? Scan my drawing, Paint or SketchFlow? No, Balsamiq Mockups!
On the web site there is an introduction video and a web demo you can use to get a feeling of the tool. The best part is that I was able to create a simple UI including the changes I was planning do to in less than 5 minutes and downloaded the PNG directly from my web browser!
It works really well to create low-fi prototype of fat clients, web apps and even iPhone UI. Additional controls can be downloaded provided by the community. Finally Balsamiq Mockups can be embedded in other tools like Confluence, JIRA, FogBugz and XWiki.
If I ever need to do a lot of UI prototyping I think a 79$ for the full version will be really well spend.