Js-visit-tracker is a project mainly written in ..., it's free.
Tracks visits to a subdirectory or a subdirectory regexp, and executes a callback function
Purpose
The purpose of this js drop-in tool is to track unique visits to a specified subdirectory's pages and execute a callback function, after a specified number of unique visits has been reached for pages in that subdirectory. The user can specify a unique sub-directory, or a regular expression that matches the set of subdirectories to include.