WindowMover is a simple utility that moves the active window from one screen to another. If you use multiple screens with your Mac, this was designed for you.
With a keystroke, you can quickly move the current window to any screen. Personally, I use this to quickly move windows to or from my primary display which sits right in front of me, to my secondary screen just to the right.
You have to compile and install this by hand right now. You'll first need to install MacRuby, and then build in Xcode 3.1.