Kattive is a captive portal with an interesting interface, written in Perl and completely Web-based. It interacts with iptables or whatever you want. It's template-based. It works as a transparent rule activator and as an Internet Point interface. It has many features like schools/classes administration. If used in a hotel, you can count the time for billing.
YFi Hotspot Manager is a modern AJAX Web front-end for FreeRADIUS 2.x. It has support for vouchers, permanent users, billing post paid, and pre-paid. Multiple languages are supported. Statistics can be generated. It works with CoovaChilli and Mikrotik. It runs on Ubuntu and CentOS. It has Google Maps integration. Third party systems can be integrated. There is thorough documentation.
The Wifidog project is a captive portal solution that has three main functions: location-aware delivery of internal or external content, authentication and authorization, and centralized network monitoring. Most of the contributions to its development came from wireless community groups, so this is the model with the most support. However, Wifidog has an extremely generic design. As such, it is now used by several other types of organizations. With relatively small amount of development it can also be used for many other business models.