I just read about this at TechCrunch, and with GeekMeet 2007 up for discussion, I thought that everybody who reads this could use it. [hint, hint] AND! There's an RSS feed (for the non-luddites :P) for the cities you put in. On first glance this is very neat, I'll be interested to see how the trends go.
Airfare Predictions, Find Cheap Airline Tickets - Farecast
And in an homage to the BSers that read this blog, another interesting site from Techcrunch.