Alexa, which is owned by Amazon, has been around since the infancy of the Internet — first it was a search engine and online archivist, and now it's all analytics, all the time. So, as you might guess, Alexa is hanging on to a lot of data. It provides traffic data and other statistics on 30 million websites, and the company site attracts millions of unique visitors every month [source: Alexa]. The company's main claim to fame is the Alexa ranking, which is a comparative traffic listing that's updated every day.
Alexa is seen as a decent source for site comparison and general traffic trends, but there are questions about its accuracy. The company is tight-lipped about exactly how it gets its data. At first it was primarily through tracking users who downloaded its toolbar. Now it's from a "global traffic panel" of people who use a certain browser extension and from sites that choose to be monitored through the installation of an Alexa script. So it's hard to tell just how solid of a cross-section is being examined. Smaller sites don't get the detailed treatment of larger ones, so if your site doesn't get massive amounts of traffic, Alexa might not be very useful.