Looking at Mark Shuttleworth's mockups for Maverick Meerkat made me wonder about the tool he used. Luckily, the tool was mentioned in the images -- Balsamiq.
Balsamiq is a nice tool. You can try out a Flash version for yourself. There's a pop-up after five minutes, but you aren't prevented from continuing the trial, which is nice. While the application is designed to help teams work together, and the website touts "collaboration," Balsamiq doesn't appear to have real-time collaboration.
Balsamiq is $79, which is almost nothing if your business is design. The Linux version is available for demo download here.