>I just got a 21" screen for developing and although it is much nicer than my former 17" screen, I still seem to want more real-estate.

what resolution are you using? If you used 1600x1200 (feasible only on large monitors) you would have the equivalent of 4 800x600 desktops.

If your company is generous enough to give you a 21" monitor and you aren't impressed enough with that and want more, why not just get a second computer?

A 2 or 4 port KVM switch is far less than a second 21" monitor, and you'd have the spare processing power and inherent multi-user test environment. (you might need a small ethernet hub to share your ethernet port too $30-100)