www.camelcamelcamel.com is a website which provides historical Amazon.com product prices. If you like a product on Amazon but are not sure if you are getting a good deal, you can check out the historical price of the product by pasting the Amazon URL or the ASIN (e.g. B00746Z6RK).
For example, the 13.3 inch 128 GB MacBook Air is currently $949 on Amazon.com but Camel3 shows that it was $50 lower (i.e. $899) a month back on June 19, 2014.
The other nice thing is that you can put in your target price and get alerts when the current price reaches your target price. You can also install a browser plugin (see above screenshot for Firefox plugin) so that you don’t have to leave your browser – very convenient.
I wish they had an iPhone app though.