Website ranks are calculated based on the traffic they receive. This rank in-turn reflects the reliability, popularity and quality of the website. Alexa is one of the top web traffic data providers collecting, processing and representing several factors of a website like Global rank, Country specific rank, site speed, bounce rates, visitor information etc. Alexa has cool extensions for Chrome and Firefox.
Once you install the Chrome extension or Firefox Addon, you are good to go, you can see ranks of websites as you browse.