How can I make focus follow the mouse cursor? I will often click on a button expecting it to be clicked but instead all that happens is the application it is in becomes active, and I have to click again to actually click the button.  It would be nice if this second click wasn't needed, which leads me to my question:


  How can I make it so that when I move the mouse cursor over an inactive window, it becomes active?
This is freely possible for the Terminal and X11 :

defaults write com.apple.Terminal FocusFollowsMouse -string YES
defaults write com.apple.x11 wm_ffm true

Or, OS-wise, with a utility that seems to fit your needs, called MondoMouse.