Travel Zoo

Travel Zoo will e-mail you the best travel deals in your area each week. I've gotten a heads up when a particular airline was having a massive sale and, thanks to Travel Zoo, bought my ticket (I believe it was BOS-JFK $29 each way) before the deal sold out. They also inform you of hotel, entertainment and other travel-related deals.

Yapta

Yapta is a travel site that has saved my fiancée and I over $300.  Once you book a flight, join Yapta for free. Then enter your flight information on the site. If the price of your flight drops, Yapta will notify you via email.  Certain airlines (like JetBlue) allow you to call and get a credit for the difference towards your next flight!