The top ranking system on the net. Having a good Alexa rank can supremely affect the sponsors who are willing to invest in your site. Installing an Alexa badge on your site will help keep tracking and statistics more accurate.
The worlds second Largest tracking system giving you more thorough details about your sites audience and popularity for free.
Snipesearch.co.uks on spin on quantcast which includes a rank badge similar to alexa but a measuring system closer to Quantcast.
A measuring system that allows you to not only track the visitors to your site but also the origin of the user.