Sub natted WISPr Sites
One of the fun things I've gotten the chance to work on are WISPr sites. These are sites where a local device, usually an AP in my experience, but potentially a router or other captive portal system, puts network-joiners into a walled garden in which they can only access an External Portal Server. This server then works with the local infrastructure through some kind of controller intermediary to grant access based on purchases or any other factor it chooses.