I myself am fairly new to the setup (within the past week). I find it most useful for coding or web browsing, because that's when you can really get a lot of windows going. Other times, I find myself dragging windows onto the second screen just to justify having it.
As for gaming, OpenGL doesn't seem very happy about dual monitors (I have to disable "Extend my Windows desktop onto this monitor" the second to play OpenGL games), but some Direct3D games are great about it. Star Trek Armada, for example, uses the second monitor with a cool "find coolest camera angle possible" algorithm to provide a cinematic view while you play. It'll get you assimilated every time if you get distracted by it.
Also, while using anything that goes full-screen on one monitor, clicking on the desktop in the other monitor causes the full-screen app to shell out. So don't expect to be able to check your mail while watching your peons build a farm or something like that.
Let me know if you find any more cool uses.