does one simple thing: run it and it puts your display immediately to sleep, no fuss, no muss. Of course you could turn manually down the backlight, go to the Energy Saver Preference Pane, or even put the entire computer to sleep, but Sleep Display shortcuts all of this: run the application and the display goes dark. Hit a key or move the mouse and the display wakes up. It's perfect for use with Quicksilver. This is a great boon to portable users especially, since the display tends to be the biggest drain on the battery (and running it doesn't affect your normal Energy Saver settings).
On my iMac I've had some problems with automatic display sleeping (i.e. it often simply doesn't work), but this little application does the trick just fine. Another great thing about Sleep Display is that when you run it, it sleeps the display and then quits, so it's not running all the time. I think the icon (right) could use a little work, but since I hardly ever see it, I don't think that's a big deal.
There's also a Dashboard widget
version if you swing that way. Sleep display is a free download for either the application
or the widget
[Via Infinite Loop