Loggin a user across different domains

[mike]

two years ago I had to design a system to share authentication data across multiple domains, all of them shared the same server/db. I was able to pull this off with a complex system of cookie sharing which, to date still works.I'm now in the process of redesigning the system and I was wondering if there are better ways to achieve this without having to write cross domain cookies.Basically the system MUST do this.Once logged in one site the user must be logged in all of the other site seamlessly, not only following a link, but even by directly writing the domain name on the address bar.To my knowledge the only way to achieve this are cross-domain cookies, if there are alternatives please tell me.Thank you very much