This is a simple little two-bit Excel trick that I find myself using all the time, particularly when formatting worksheets.
In Excel, Control+Y is the 'other half' of the Undo/Redo pair. If you undo an action and want to redo what you just undid, Control+Y undoes the undo, so to speak. However, if you haven't undone anything, and there's nothing on the redo queue, Control+Y repeats the last single action you took.
Repeatable actions can actually be quite complex. For example, opening the Format Cell dialog box and applying a format counts as one repeatable action, regardless of how many format attributes you change. Once you make that format change to one cell and before you do anything else Control+Y has become a key that applies that specific format change to as many other cells as you like.
In a sense, Control+Y is a command that's eternally bound to a simple macro that Excel keeps updating with your last action. If you plan your work to group actions together, this 'automatic' macro can save a lots of time.