We're running two twin monitor setups at home and I have a big single at work.
My gf runs her video-editing so twin-screens is vital to give space for the preview, timeline, clips and various other effects windows.
For me I originally did my web-editing on one with the preview + IRC on the other so I could keep in contact with the mod team and work at the same time. Nowadays I find it useful for referential surfing where I keep various windows in the usual manner on one monitor while I have the comparison window on the other.
I'm still not happy the screens are only capable of 1280x1024, especially for the film editing, but there are no good-value 19" TFT's capable of 1600x1200.
For work I could really do with twin's (current 1600x1200 on a 19" CRT) so I can keep track of my servers, monitor user usage and do my usual admin work but I got far more important things to get sorted first!