Last time I found interesting tool called Balsamiq Mockups. It is a Flash based mock-up toolset for rapid prototyping UI designs. I’ve been using many different applications for gui prototyping but this one is really cool and useful. It is simple, powerful and flexible, this is exactly what gui prototyping tools should be. Of course there are many more complex and “enterprise” (my favorite word) applications, like Axure - which I’m still using or very excting Adobe Thermo. But again, sometimes we need someting very simple and handy like Balsamiq Mochups. So go to their web site or try it now here. There are also demonstration sreencast.