Monoff (updated August 3, 2011)
Monoff will save the world! Ok that's probably an exaggeration, but it can save a bunch of energy by turning off your computer monitors. Put it on your desktop, click it, and your monitors will turn off. Move your mouse or hit a key and they'll come back on.
If you need more control over delays before your monitors turn off you might want to check out Sleeper.
Advanced usage:
You can also make it turn off your monitors after a certain amount of time by adding a commandline parameter. For example, if you want it to turn off your monitors in 60 seconds, you'd run it like this:
monoff -delay=60
Or you can make some specific monitor turn off. Actually in this case you're not really turning off the monitor, you're just blanking it out, I don't think there's any power savings:
monoff -monitors=2
Turn off more than one monitor (but not all monitors) like this:
monoff -monitors=2,3
Turn off monitor 2 after a delay of 5 seconds:
monoff -delay 5 -monitors=2
Don't worry if you have no idea what the commandline stuff means, just click the monoff icon and your monitors will turn off and the world will be saved.
And you can use Sleeper to do all this too. In Sleeper, set check the box next to "reset countdown when mouse or keyboard is moved" and it'll behave as a screensaver that's a lot more reliable than the screensaver timer built into Windows.