Capital_C - 7:22 am on Dec 20, 2008 (gmt 0)

When I log into Gmail, Blogger.com also is aware of my logged in session.
Same with Yahoo-Mail & Flicker.

How do they do this? I assume with Cookies, but isn't it pretty difficult to allow one domain to access another domain's cookie?

thanks
(sorry if this is the wrong forum-subject, I really wasn't sure but figured it applied equally to all server-side languages and maybe occurred more on the browser side)