NoCat & NoCatAuth at
www.nocat.net a little out of date
wifidog at
http://dev.wifidog.org/
ChilliSpot linksys firmware (open source) -- build in your own Linksys AP
And the next ones from wikipedia
# FirstSpot - commercial Windows based hotspot software
# Hotspotexpress - software based for Linux platform (commercial)
# WiFiDog Captive Portal Suite - small C based kernel solution (embeddable)
# Wilmagate - C++ based and is executable both in Linux and Windows/Cygwin environments
# pfSense - FreeBSD 6.1 based firewall software derived from m0n0wall
# SweetSpot - Linux user-space, layer-3 daemon (open source)
# Air Marshal - software based for Linux platform (commercial)
# "TocToc" - software based for Linux platform generated by the Technical University of Valencia (open source).
# "DNS Redirector" - software based for Windows servers, now freeware.
About the dificult, I spend a couple of hours with NoCat & NoCatAuth on a old dell notebook without problems .... the others, don't know....