Alexa

Alexa, dubbing itself as "the Web Information Company", is a web analytics company that is most well known for their Alexa Toolbar that tracks and records web metrics and surfing patterns for a database that supplies information feeding into their primary services of Site Info, Top Sites and Site Audits.

More information about them, from the website:

Alexa is the leading provider of free, global web metrics. Search Alexa to discover the most successful sites on the web by keyword, category, or country. Use our analytics for competitive analysis, benchmarking, market research, or business development. Use Alexa's Pro tools to optimize your company's presence on the web.

Sometimes useful, but mostly annoying. Better to use the website search than be spied on with a toolbar installed.

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