You can charge clients for monitoring web sites for them, simply keeping an eye on certain Web sites. The information featured, number of 'hits' (people visiting the site), and changes made. You can present it as a report which shows how the company could update their own site to make it better than their competitors.
You can charge hourly for this service, plus telephone expenses etc., but you need to keep your customers informed of what you've been up to.
Use targeted e-mail, letters, ads in the various Internet magazines, plus your own Web site, to attract custom.
There are many services available to help analyse different aspects of web site performance. You could subscribe to these services and then resell the information to others.