Alexa announced today that their algorithms have changed. Alexa is a third party website ranking service. Alexa is free and until Compete and Quantcast came along Alexa was the only game in town. You may see your Alexa ranking have a sharp increase or decrease today.
Alexa first received their data from users that installed the Alexa toolbar. Many did not believe the rankings would be accurate if the toolbars were not installed in a large enough population then the sample size may not be valid. Currently they state that the new rankings also take data from other sources.
Take a look at the new Alexa rankings here.