I have three monitors, unfortunately one of them isn't 1080p so cant run Eyefinity. And I suspect my GPU would die if I even tried .
On the right hand monitor I have some general system meters. RAM, CPU and GPU usage with some temp monitors for the CPU cores and GPU (Rainmeter, awesome thing ). Left hand has more of a general usage, date, time calendar type stuff.
Occasionally when gaming il have a walkthrough or something on a different monitor.
But the main advantage of multiple monitors is just more screen space. Rather than trying to cram two webpages and a word document onto the one screen, only so many pixels each. Spreading the windows across the monitors just makes it so much easier to do productivity tasks. Video editing I have also found it to be useful, spreading the resources panel and preview window to the left and right monitors to give greater room to the timeline and effects panel, just makes it so much easier to work with.
EDIT: perhaps I should mention that while writing this, I have been watching a Youtube documentary on the left monitor. Cant do that with a single monitor, trying to fit a movie and a web browser on the same screen just results in a horrible experience.