Alexa.com - Free service that assesses the traffic of any website, offered by Amazon inc.
Ranking.com - Online popularity of the most visited websites.

Quantcast.com - Free traffic assessment, but reliable only for the medium & large US domains & websites.

Compete.com - Assessment of traffic for any website.

Nielsen-netratings.com - Statistics about internet usage & websites visited in selected countries. Great but very expensive paying version.

 

Other tools and links for domainers >>