If you're looking for a particular item to go on sale and you have the patience to wait for the price to drop, Woot is where it's at.
What Woot does is buy a ton of merchandise on the cheap, then sells it to its registered users (sign-up is free) for a huge fraction of the regular price. Usually only one item is posted each day at midnight Central time.
But they've just launched something to make all of our lives easier — wootoff! Just follow wootoff through Twitter and you'll be able to stay up to date with all Woot sales since there is a constant stream of items. The way in which you'll know that there is a wootoff is by the yellow flashing lights (when you click on the product link via Twitter). There's also wootshirt and wootwine as well.