Blue dot is a free GPL licensed Perl CGI tracking and web site activity measurement script which generates Apache combined style access log files. These log files can be analyzed with most standard log analysis tools to track site's popularity, referrers, hosts, etc.
The logging is based on inserting a small piece of JavaScript or SSI code into every web page. This code requests a very small blue dot image from a server where the call is logged.
Blue dot can also be configured to set and log session and persistent cookies. This can be used, for example, to track pay-per-click search engine ROI.