displaykeyboardmacoswindow-manager
Is there a free app that I can use to move a window from one monitor to the other monitor (assuming only two monitors) via the keyboard? I shouldn't have to use the mouse at all (e.g., for selecting the window or the monitor, or for dragging).
I know this can be done in Linux using compiz; I'm surprised there isn't a (free) option for OS X.
You don't need to worry about Spaces for this. What you need to do is go into:
System Preferences --> Displays --> Arrangement
Drag the icon representing your external monitor to the top of the icon representing your macbook monitor:
You can also drag the bar representing your menubar/dock to whichever monitor you want to be your "main" display.
Pressing the shortcuts for changing desktops while dragging a window should still work, but the shortcuts have to be enabled in System Preferences.
SizeUp supports for example assigning ⌃⇧→ to moving a window to the next desktop.
Best Answer
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 > Accessibility > Enable access for assisted devices
As per comments: The accessibility option in Mavericks is now in
System Preferences > Security and Privacy > Privacy > Accessibility