Why developers at startups use 3 monitors?
AndrewO: 1) Why? As others have said, enhanced productivity.  I usually keep API docs, examples, and a terminal with autotest on my secondary.  Also, in my case, my machine is a laptop (I use that screen as my secondary) and knowing I have a 24" screen waiting for me is incentive to come into the office instead of telecommuting or siting in a coffee shop :).2a) How? Been awhile since I've had to deal with installing new hardware, but as far as I remember, you can install new video cards and it'll register fine (or get a card that has two DVI ports).  As for OS support, I think Windows does it (although I don't know if you have to have a certain edition), Mac supports it on most models out of the box (in the past you had to do a software hack on iMacs, don't know if that's still the case—other than that completely painless), and I know it's possible with Debian (probably most other Linuxes as well).2b) What's it look like? Most current hardware will support spanning across multiple screens (OS support: see caveats above).  In the past, some video adapters didn't have the horsepower to do anything other than mirroring (where the screens show the same thing), but all of the ones I've dealt with recently can (again, YMMV).  Mirroring is still useful when hooking up to a projector, but beyond that, spanning is the way to go.