Do you want to spy (or ‘research') on your competitors to know how much traffic their sites are getting? Now there are a bunch of traffic estimation services (like Alexa, Compete, Quantcast, etc.) from where you can obtain statistics (page views + unique visitors) of almost any website for free but is the data accurate? I did a rough comparison using actual data from Google Analytics data and here are the results: 1. Alexa - Alexa won't tell you the actual page views of a site but an estimate of the percentage of Internet users that may have visited your site during a given period. Now assuming that the number of Internet users did not changed drastically in the past 6 months or so, a site's reach as determined by Alexa can be proportional to the unique visitors. Verdict: The Alexa chart pattern for my blog is fairly similar to Google Analytics which is actually very surprising. 2. Trends for Websites - Google Trends uses data from Google search, Google Analytics and other third-party market research services. Since Google search is the largest source of traffic to most websites (especially those in the tech domain), Trends for Website can give a fairly accurate estimate of site traffic. Verdict: Google Trends estimation of "daily unique visitors" can get very close to actual numbers but the only problem is that Trends doesn't provide data for low-traffic websites. 3. StatBrain - This is the only service that claims to provide a count of daily visitors to any website. StatBrain does so using data from a number of external sources like Alexa Rank, backlinks in Google, location of web server, etc. 4. Compete - You can use Compete to estimate the total US traffic for nearly every site on the web. Compete uses data from ISPs, their own toolbar and other data source to calculate the unique visitors and hits (or pageviews) of a site. Verdict: Very close to actual data but limited to visitors from US only so you won't get the true picture for an international website. 5. QuantCast - Like Compete, QuantCast too gives an estimate of hits and unique visitors along with visitor demographics. Verdict: Data is not available for all websites and also less accurate than Compete 6. Google Ad Planner - This is my favorite tool that is both accurate and comprehensive. You get to know the traffic of any website in a particular geographic region (or worldwide), average number of visits per visitor, total page views, unique visitors and the broad category of that site. 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. Verdict: Highly recommended. 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.