See average daily visitors of other websites with http://nextpr.info It is a completely free service and gives very good estimates of other websites traffic. The data is updated monthly. In addition you can also see page rank history, current page rank and future page rank. The page rank prediction is very accurate as it is based on the number of back links. The website traffic data is also collected from reliable source. The tool also shows whether pagerank is artificially inflated. So when you are buying a new domain or a website, be sure to check out the traffic history and reality of page rank using this tool. Don't forget to share it with others on facebook, twitter and google+
If you want to have an idea of traffic of other's website then you can use Alexa which gives an estimate traffic of other users. You can use alexa toolbar to increase your own alexa ranking.