I have tried two or three screens before, but just got irritated by the noise.
I check email and hn on breaks. They don't have dedicated screens during work periods.
I don't know if there is a similar program for the win/lin world.
The downside is, changes to the underlying layout require a manual refresh.
This works even with several computers, over a network.
btw. xrefresh also works over network and does "soft css refresh". Some disadvantage of that emacs system is that it bypasses whole server-side stack (imagine PHP or HAML/SASS)
I use 4 monitors. The 4th one is great for not having to manage IM window positions or look at them when I'm working, since it's outside my normal visual focus, while letting people still contact me asynchronously for things they need from me.
The two vertical make it easy to compare code and work on two files side by side. The first horizontal is great for checking UI work in the browser.
I worked this way for about 2 or 3 months, and loved it. I got accustomed to switching desktops and console sessions and toggling about.
Now I use a Dell Lattitude with a 15" 1920x1200 screen, and a 24" 1920x1200 monitor attached. And I have my Windows and Mac boxen sharing a keyboard (Endurapro 104 "clacky" keyboard with built-in clit mouse) and 24" 1920x1200 monitor as well.
The second monitor has its advantages (I can keep a nice busy set of IRC channels always in view) but I still tend to work via desktop toggling. Increasing familiarity with screen (and now tmux) is playing a role as well.
Btw using an external monitor I lost the "portability" benefits of the macbook, and I'm used to work on the balcony when it's sunny :)
This poses another problem btw, the contrast is too low with the macbook display.
I also have an old laptop set up to the left of the left monitor that shows the key metrics for my startup (user count, etc.) Sometimes it's bad to have this right in your face, but usually I find it motivating. :)
Someone else mentioned Synergy; Windows-only devs can also check out InputDirector: http://www.inputdirector.com/