This tip is great for use in MS Word or Excel.

Hitting the F4 key will repeat your last action in either of these two programs. For example, if you have inserted a row or column in Excel using the Insert menu, hitting F4, will insert a second row or column without using the menu.

It is also great for formatting text in Word, for example, bolding, underlining, text color, or italicizing, etc. Just remember it repeats your last action only.