There are many ways to track the competition, from reading newspapers and magazines to checking them out on Alexa or other web stats and analytics sites.
Here are some articles referencing how to track the competition in a variety of ways including traditional media, social media, analytics, and legal spying.
Comparing your website to others means tracking their activity compared to yours. This might involve using an online tool that compares your site to theirs, or by checking each site individually and comparing notes. Some of the following will compare one site to another testing for a variety of data such as PageRank, keywords, traffic, and incoming links. Others will focus on a single aspect such as keywords or unique visitors. Use a variety of them to get an overall perspective on how your site compares to others.
Note: Google removed their PageRank data from the public a couple years ago. Online tools based upon Google’s PageRank data may no longer work or be supported. Please let me know if any of these are no longer active.
The following are SEO and analytics testing tools that cover specific types of testing. For example, BuiltWith tests the site and reports back with information on how the site was build, the publishing platform, supporting technologies, and other information about how the site was built.