Since I already published a Windows shortcut today, here’s a bonus for you Mac users, with thanks again to Kevin Fox.
Suppose you’re using a Mac and you’re creating a selection rectangle in Photoshop. Normally, moving the mouse changes the size of the selection rectangle. If you hold down the Spacebar, suddenly you’re now moving the origin of the rectangle instead. (Practice this a bit to get the hang of it.)
Many of you are old hands at Photoshop and already knew that one. (You may have known it works in Windows versions of Photoshop as well, plus many other image editors on both platforms.)
Okay, so here’s the new bit: If you use OS X, as of 10.5, when you press Command+Shift+4 to capture part of the screen, you can now use the Spacebar trick to switch between resizing and changing the origin point.