Captive Portal 101

http://en.wikipedia.org/wiki/Captive_portal
The captive portal technique forces an HTTP client on a network to see a special web page (usually for authentication purposes) before using the Internet normally. A captive portal turns a Web browser into an authentication device

.Software captive portals

Air Marshal, software based for Linux platform (commercial)

Amazingports, linux based software with integrated billing and payment implementing
Service Oriented Provisioning, free and commercial

ChilliSpot, open source Linux daemon [abandoned]

CoovaSpot, open source Linux daemon based on ChilliSpot
DNS Redirector, Windows based software solution (commercial)

FirstSpot, Windows based software solution (commercial)

HotSpotPA, open source Linux daemon based on OpenWRT, OpenVPN, and ChilliSpot
m0n0wall, FreeBSD based firewall distribution
PacketFence, Linux based Network Access Control software featuring a captive portal (open source)

pfSense, FreeBSD based firewall software derived from m0n0wall
Sputnik, Software as a service solution (commercial)

Untangle Captive Portal, Firewall featuring Captive Portal

WiFiDog Captive Portal Suite, small C based kernel solution (embeddable)

Wilmagate, C++ based and is executable both in Linux and Windows/Cygwin environments
Zeroshell, Linux based network services distribution