The extension must keep track of cookies that are in use (when a website is visited), and allow the user to remove cookies that haven't been used for some time (eg: a week).
An alternative implementation would be to limit the max lifetime of cookies, and refresh it as they are used.
This extension must be compatible with Chrome, and optionally with Firefox/Safari. It must have a Preferences tab to see cookies sorted by the time they were last used, and configure the retention period.
The project must be released under an open-source license (after the payment, of course).