Paid webuser subscriptions for MODx. Allows you to use FoxyCart subscriptions to manage webuser permissions. Useful for creating a paid members only section, paid blog subscriptions, paid premium content subscription, etc.
Could be modified to use a non-subscription product as well (one-time payment for unlimited access).
Paid Webuser Subscriptions / Permissions in the MODx repository
Support thread in the MODx forums