It's sort of sad that game developers still refuse to admit that second monitors are relatively common anymore. One of the few games that I remember even supporting dual monitors was Supreme Commander. Honestly, it made the game feel a lot cooler than it would've otherwise.
With a bunch of hassle, you could do splitscreen Left 4 Dead across a dual monitor setup. I used to play online with my roommate all the time like this. However, it wasn't officially supported (to say the least) and was sort of a pain to get setup. With minimal effort, that could've been a real feature, and a lot more people could've taken advantage of it....
Any sort of second monitor use would be ace. Typically, the best use would be: big, blow up map, and some basic/important stats.