I was checking some apps I wrote in fullscreen mode with Forefox 3. They become interestingly integrated into the desktop.
There were some extensions on Firefox 2 to do something like that, but Firefox 3 does it natively and having the toolbar and menus disappear completely when in fullscreen mode makes it nicer to work with.
I decided that I’ll use more applications — Google Reader, WordPress, etc. — in fullscreen mode. It feels more comfortable.