For a long time I had the ghetto equivalent of dual-monitor. There was a 15-inch laptop open on my desk, next to a nice square 19" Dell LCD ($30, thank you bankruptcy auction) for my desktop box. And there was this cool background service called Synergy which ran on both, so that whenever the mouse cursor on the laptop would move off the screen, it would seamlessly jump over and start controlling the Desktop machine (via Ethernet), along with some niceties like sharing the Clipboard.
But then the laptop got moved to my parts drawer, and I purchased a 24" 1920x1200 monster to compliment my other screen. My only regret is not buying two.