I was using ShiftItShiftIt for ages.
However, butI thought it didn't support dealing with multiple displays. So, so I started looking for alternatives. I installed SpectaclesSpectacle, and changed the key bindings to be the same as ShiftIt's.
Then, and then when I wanted to deinstall ShiftIt, I found out it does actually support moving windows to different screens, using: CtrlOptionCommandN
So I uninstalled Spectacles; still a happy ShiftIt user.