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.
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.
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.
Verdict: StatBrain says that Digital Inspiration gets around 33k unique visitor per day which is definitely on the lower side but still not a very bad guess.

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 (at least for Digital Inspiration) though the situation may be different for quantified sites.

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.