Captive Portals allow you to control access to the internet from your Wi-Fi access network. They work by forcing Wi-Fi clients to visit a purpose built web page designed for authentication, accepting terms, paying for service, etc. Captive Portals can be home grown using open source software such as CoovaChilli, PacketFence, Freeradius, MySQL, Apache and the like or they be purchases as a service from companies like Purple Wi-Fi, Cloud4Wi, So Wifi and many more.
A current trend in the paid services is moving away from the 'pay for Wi-Fi' model to a social Wi-Fi model that captures a rich set of data from the user in exchange for internet access.
What success or challenges are you having with Captive Portals in your networks?