The Tip: gv is a nifty little gesture in vim that allows you to reselect in visual mode the last visual mode selection you made.
Usage: Highlight a block of text in visual mode, then exit out of visual mode. Hitting gv will reselect it.
What Is It Good For?: It is rather nifty if you use your keyboard as hyperactively as I do and manage to deselect a visually selected area unintentionally or yank a piece of text, put it on another buffer and then go back to your original buffer and want to quickly select that piece of text again.