Balsamiq Mockups is a UI tool used to create ... UI mockups. Very useful.
There is a desktop versions as well: both for Windows and Mac. But none for Linux.
So, I've installed Wine and Q4Wine on my Linux machine; afterwards, I've installed Balsamiq Mockups Windows version (as a plain Windows application using Q4Wine). And the result: Balsamiq works perfectly on my Ubuntu Linux machine.