In Today era there are a bunch of sites are present which provide traffic estimation services (like Alexa, Compete, Quantcast, etc). By using these services you can obtain statistics (page views+ unique visitors) of any website for free.
1. Alexa - It provide you an estimate of the percentage of Internet users that may have visited site during a given period.
2. Trends for Websites- Google Trends use the data from the Google Search, Google Analytics and other Third-party market research services. Trends For Website can give a fairly accurate estimate of site traffic.
3. StatBrain - This service claims to provide a count of daily visitors of any website.It use the data form a number of extenal sources like Alexa Rank, backlinks in Google, location of web server, etc.
4. Compete- For estimate US traffic for website you can use it. Compete use data from ISPs, their own toolbar and other sources to calculate the unique visitors and hits of site.
5. Quant cast - It is the service which provide an estimate of hits and unique visitors along with visitor demographics.
Here's how you can use Google Ad Planner to determine the traffic of any website:
1. Click the "Begin Research" tab in your Ad Planner dashboard.
2. Type the URLs of all websites that you want to research in the "Sites Visited" section of "Online Activity"
3. You'll see that all those sites now appear on the right side. Click the name of the site to view the traffic and demographic details of that site.
Bonus Tip: If you need to check the rankings of any website on multiple services in one go, try Dataopedia or Attention Meter. Both these services offer traffic graphs from Alexa, Compete, Quantcast, etc. on the same page.