I could not live without my second monitor. Even though it is only a 17", it is a life saver. You know when you leave the house without your phone? That's how I would feel if I didn't have this second monitor.
I have a single 24" at home, and although I hardly do any business work at home, it is a PITA. First world problems.
Edit: OK, now we all get trumped by dodecagon boy here.