extra space to throw windows in while I'm not using them
Not using them? Surely you'd close them if you weren't using them?
Personally, I use the main screen as my "active" monitor and the secondary as a reading and reference screen. Or for keeping an eye on something while doing something else.
You can, of course, get multi-monitor wallpapers to span all your monitors.
I push my taskbar to the secondary monitor to give me more space to work with.
I guess if you were that way inclined you could two-box an MMO, one on each monitor (yes, EVE, I'm thinking of you).
I do, however, get fairly odd results with games and multiple monitors, largely, I suspect, because they have different sizes and resolutions. The secondary is often either blacked out, forcibly reset in resolution, or otherwise mucked with in unhelpful ways. Also, with only two monitors, there doesn't seem to be much point in trying to find a way of spanning both screens with most games as having extra view only to left or right just feels weird (if it's supported at all).
Mostly just find dual monitors useful for software development...