The Alexa toolbar, from amazon.com (and by the guy, Brewster Kahle, who created the internet archive too, by the way) is one way to get some idea of how popular a web site is (the toolbar shows the web site rank for each site you visit). It is very inaccurate, but is free, and easy, so it is used. Alexa Toolbar and the Problem of Experiment Design shows why data based decision making is not the solution to all your problems:

As Dr. Deming said best efforts are not enough, you need to know what to do first. The same holds true with data, first you need to have useful data.

Related: Manage what you can’t measure - podcast interview: Brewster Kahle - Gladwell and more IT Conversations