I have recently come across an impressive new web site called Farecast which goes beyond what other travel sites offer. While Orbitz and Expedia will allow you to find the best price for a flight on any given day, they do not show you what that flight cost yesterday or a week ago. Farecast will show price history for your flight and gives advice on whether to buy the flight now, or wait for prices to drop. It is still in a private beta and currently only lets you search trips from Boston or Seattle, but I am looking forward to using the full service. Contact me if you want an invite to try it out. Link

Farecast’s price history chart:
Farecast