Alexa is a website which gives out the ranking of all websites on the net including the blogs. The ranking can be seen in terms of overall worldwide ranking or country wide ranking. The Alexa ranking is highly believable and ad companies usually take the help of this ranking to decide which website should get their ads.
You can also get the comparison ranking on Alexa. Just enter the URL of your website and the URL of that of your competitor and see how your website id performing.