We all wonder what will be the optimal time to buy an air ticket. It typically is a couple of weeks beforehand, but if you have already missed that, there is this new website http://www.farecast.com which helps.
The site gives a graph of fares for your destination across a few months, and gives you recommendations on whether they are expected to rise or fall. They recently added a lot of cities (were only seattle previously), and chances are you will find your city there.
A really useful "web 2.0" website which is not about social networking :).