At home, I've got two 24" widescreens on articulating arms; I've ordered a third, so we'll see how that goes. At work, I've got two (nicer) 24"ers; having two monitors was literally a precondition of employment for me, as I'm just not productive otherwise. I've also got a little 7" USB monitor where I keep my IM windows, so even when I've got a browser on one screen and a full-screen app running on the other, I can still see urgent messages like,
BigBossMan: "Wait! The customer doesn't want (thing you were about to do) done!"
For those without the practical ability to do three monitors given the constraints of their budgets/video card(s)/desk space/corporate policy, the USB option is actually remarkably powerful. It'll even run 3D stuff. You can find larger ones, as well, but the 7" worked quite nicely for me since it fits in a nice, cozy little spot underneath the main monitors.
Here's a picture of my setup at work, slightly edited for your viewing pleasure: http://img269.imageshack.us/img269/4761/anonythree.png . Quality was already crappy enough, so yes, I used a PNG where a JPEG might otherwise have been more appropriate. You'll live.