Farecast

Farecast, launched earlier this year, is an airfare search engine which uses data-mining algorithms to predict the future behavior of fares. When you enter your travel dates and airports, Farecast will tell you, with what percent confidence, the behavior of those fares over the next 7 days. Then they will issue their Tip: Buy or Wait. Other features on the site include a spectacular graphing output on When to Travel, Where to Travel, and Length of Travel - all based on fares. The current release (Beta) supports roundtrips between 55+ US cities. The next release promises an increase in the available cities and new functionality (such as multi-city trips). Farecast offers, to the general public, technology that has been a closely held secret of airline companies, college grad students, and Las Vegas casinos. I can't say enough for the site - spectacular.