I buy a lot of stuff from Amazon.com
Recently I discovered camelcamelcamel.com where you can enter in the item you'd like to watch the price for on Amazon.com - if it's already been "researched" before (aka someone else has searched for that item) you'll get to see a chart of the prices throughout the previous months. If it's never been researched before, they'll create a new search for it.
Either way, when you've found the item you want to watch, just type in your desired price and email address and you'll be notified when the price reaches your target price! I've got a few things I'm watching already.
The nice thing about this is that you can watch items of your choice without signing on for an account (you'll just have to watch them individually with the link they give you), or you can create an account and see all of your watched items on one page! When you create an account, I'm pretty sure you can also "price watch" items on other sites such as Best Buy and Zzounds!