I am the owner of a popular file sharing site and I am looking to set up a code on the rar files that people download. In the rar file there will be a "read me file" with instructions on how the user can download and get the password to open the file. The user will then be prompted to follow the link back to my site whereas he/she will have to download a toolbar to get the code which will open the respected file.
The site runs on a xfilesharing script. Let me know if this makes sense and if you are interested in this proposal.
Look forward to hearing your response.