Pfsense is a bsd based firewall solution with built in captive portal capability. Its extremely powerful, and works perfectly for wireless hotspots. Built in DHCP server controls IP address assignment on the network, and the captive portal page is completely customizable, with capability of authenticating against radius servers or the local database. Pfsense also has built in functionality which enables you to make use of multiple internet connections (thus providing extra bandwidth). Check it out here.