alexa (owned by amazon.com) have a product called "alexa toolbar", they said
about 1% of PC in the world have this toolbar installed.
These toolbars will silently track which sites the user visits and use these
stats to guesstimate your site's traffic rank.
they've updated their algorithm i think, now it is based on a number of
factors other than the toolbar. Take a look at this page for more details: http://www.jiansnet.com/topic?id=20702
