Is there a keyboard shortcut to move a window from one monitor to another?
UPDATE: Spectacle is no longer maintaned.  The website forwards users to Rectangle, see Jack's answer or https://github.com/rxhanson/Rectangle/releases.

I just tried Spectacle, great app and it's free.
This is what it can do with the selected group of windows.

You need to activate accessibility support.
System Preferences &gt; Accessibility &gt; Enable access for assisted devices
As per comments: The accessibility option in Mavericks is now in
System Preferences &gt; Security and Privacy &gt; Privacy &gt; Accessibility