I have a new 24-inch monitor and a 21-inch older monitor. This older one is about to die on me so I've ordered a new 25.5 inch monitor to replace it. And I just decided to use a dual-monitor system after I bought a new computer last month. Still had the monitor of my old PC so now I had a dual-monitor setup. Laughed about it, wondering if it would ever be really useful.
Well, now one monitor is going to die on me and I noticed I just can't live without this dual-monitor setup! Well, almost... When I develop software, it's great to have the IDE on the left screen and the application on the right. When I go through my photo collection with Adobe Lightroom, I see a grid with small thumbnails left and a big one right. And with Supreme Commander, a computer game, I see my own troops left and the enemy troops right. When web browsing, I can have two browser pages open, each on their own screen. Or I just watch TV or DVD on one screen and do something else on the other.
Only drawback? I needed a wider wallpaper... :-)