I came by a nice little screen mocking tool. It's called Balsamiq Mockups.
The greatest thing about this tool is that your screen mockups will actually look like mockups. Making mockups with Visio or other tools will give the user a false impression of completeness. But with Balsamiq Mockups, it's clear that it does not represent the real thing... That there are still work to do.
While the result looks like a drawing, it still remains clean and readable.
This application is running under Adobe Air so it is available on any platform running Air (Linux, Mac, Windows). It is very easy to use and comes with a complete palette of widgets.
The only drawback I found was it's price... While at 79$ it is relatively cheap, as a Ruby and Java developer, I'm kinda used to free tools. However, there is really no equivalent for this tool in the free world and updates are free. So I guess this is a good investment.