Three: one widescreen 24" in the center (for code), two 19" square screens to the left and right (left used for web, right used for communication - IRC, IM, etc.). The center screen is great, because I can crank my font size up to 16pt (I have bad eyestrain problems). When I'm really working, I typically pop out Firebug and cover the right window with it.
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. :)
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/