Farecast
I found this site to be very, very useful:
They forecast airfare so you know the best time to buy. Will the fare go up? Will it go down? Now you know…sort of. They also send you email updates when the fare changes on your tracked flights.
What I really like though, is the flexible search option. You put in a destination city, and then you can adjust parameters for departure date, duration, destination, and trip length. As far as I know, they are the only site doing this. Most travel places let you search by flexible dates, but flexible destination is far more useful. For example:

I want to go on vacation over memorial day weekend, but I’m not too particular about the destination. So I got this sweet map to telling me what it would cost to go where. Pretty cool, huh?
jared said:
Mar 18, 09 at 9:18 pmBooyah. farecast.LIVE.com. Money.
scott said:
Mar 19, 09 at 5:56 amYeah, props to Microsoft, and you, if you worked on it.
Boston Boston said:
May 11, 09 at 7:58 pmI love farecast, It’s a great concept but I wonder if fuel prices are factored in?
Shouzer » Blog Archive » Italy said:
Jun 03, 09 at 8:55 am[...] I took my first vacation time of my career last week, flying to Rome for a much needed vacation. I found a great deal, and I just couldn’t pass up the opportunity. I spent about 7 days in Rome with a 2 day side [...]