Have you ever bought an item at a store, only to find out it went on sale a few days later? At some stores, you can bring in your receipt and they'll give you money back after the fact.

But who pays such close attention to sales on items they've already bought? A couple of new Web sites, that's who.

For free, they let you register a recently purchased product, and if it goes on sale, you'll get an e-mail. Check them out here and here.

Another idea used by some consumers: just return the item, then go back and buy it on sale. But remember, there's no guarantee the store will accept your return. There's no law requiring any store to accept returned merchandise, though most do.