[from developer's web site]
PunkBuster is a client/server software system that is designed to
provide an online countermeasures system against cheating at
multiplayer online games - specifically, games and mods based on
the Half-Life engine by VALVe software.

Additionally, PunkBuster has a global Clan Tag Registry system
built in to allow Clans to register their Clan Tags for protection
on monitored servers. This prevents people from wearing their clan
tag who aren't really in the clan. The list of registered tags is
stored on our publicly accessible Internet website.

WWW: http://www.punkbuster.com/