Central authentication for multiple sites

Hi, Does anyone know of a way to authenticate a person on one site and have that authentication carried through to multiple sites? Basically I'd like to have someone login on www.domain1.com and then have their login be valid on www.domain2.com and www.domain3.com ... the domain name is different so I don't see how I could use a common cookie. The 3 sites in question are hosted on a common server with a common user database if that helps but still the domain names that people access the sites with are unique. Any ideas? Thanks! -- Fraser Campbell <fraser@wehave.net> http://www.wehave.net/ Halton Hills, Ontario, Canada Debian GNU/Linux