CamelCamelCamel.com provides a historical graph of product prices on Amazon.com so that you can strategically make product purchases.
For Newegg.com price history, there's CamelEgg.com, and for Best Buy there's CamelBuy.com. Or, get The Camelizer Firefox add-on to search Amazon, Newegg and Best Buy prices at once. These sites are great resources for the price-savvy consumer. (Via Tekzilla)