Alexa
Alexa - the web information company - is a search engine that provides extra information such as traffic rankings.
An Alexa ranking is an indicator used to gauge site performance, based on comparisons drawn against other sites.
Many web professional say Alexa is heavily biased toward sites that focus on marketing and webmaster communities. But while not being highly accurate it is free.
Alexa is owned by Amazon.com.
Alexa offers a toolbar which displays traffic rank, traffic trend and reach meters directly in your browser. It's available for Internet Explorer and FireFox. It's this toolbar that they also use to gather information about traffic. Information is sorted, sifted, anonymized, counted, and computed, until, finally, the traffic rankings are shown in the Alexa service.
