If your website is set-up correctly, you should have some traffic software such as Google Analytics to measure the visits that your website achieves, how they interact with your website, and how valuable each visitor is to your specific website. This is such valuable information from an internal perspective and countless business decisions should be made from this data. How great would it be if you could reach all of your competitors Google Analytics data, and start to create learnings from their activities as well as your own website.
Unfortunately, spying on competitors in the technique above is impossible. However, there are a number of different SEO tools that can help you to start gathering some information about your competitors visitor figures. A couple of the market leaders is Alexa and Quantcast. Both of these tools provide measurable statistics and information about your competitors from trending information, demographics and in relation to the Quantcast product they try to distinguish the exact visitor numbers. Both of these tools offer an exceptional service, however, the data can never be 100% but it should give you enough details on how your competitors are faring on an on-going basis.
There are a selection of other companies that offer slightly different but still impressive results. These include Compete, Hitwise, Spyfu, Trellian, Google Trends, Doubleclick Adplanner, Comscore and even SEOMoz. All of these tools are worth investigating and together can pull together some highly useful information that can highlight new opportunities within your industry sector.