There are several free tools that can help you measure traffic data to your websites & blogs. Some of these tools can also give you comparative information you can use to understand competitors’ traffic as well.
Another part of many of these free tools is providing a small sample of keywords that lead traffic to the website that you are studying.
Membership to many of these online statistics providers would give more elaborate results. However, these tools may not be effective if the websites have very low traffic volume or are extremely new.