"gimp rectangle-select" window resize mode
For program windows It should be possible to trigger a mode that creates large handles for resizing windows like you get with the rectangle select tool in the gimp. In addition there should be large versions of the close/maximize/
The trigger needs to be fast to activate, but not something likely to happen by accident.
For mousing I suggest the cursor needs to cross a window edge fast into a window, then leave quickly by the same edge. Possibly twice.
For a multitouch screen a held touch outside the window, then a touch in the window.
There could also be an icon on the launcher that toggles this mode (probably more discoverable)
The advantage is that you have nice big targets to work with when manipulating windows, but you don't waste screen space when you're not using them.
For maximized windows dragging the menu bar down is already very convenient (at least when it's the top edge of the multi-monitor area.