Apr 23, 2012 08:51 AM|Steven Cheng - MSFT|LINK
As other members have suggested, the most common ways for sharing authentication info across different sites are using cookie. By default, HTTP cookie support sharing between multiple sites that are under the same parent/base domain name. e.g.
abc.microsoft.com def.microsoft.com xyz.microsoft.com microsoft.com
these sites can share cookie stored at the root domain path naturally. While sharing cookie across different domain will require additional work on the sites that need to share cookie between each other. Here are some web reference:
#Sharing Cookies Across Domains
#Forms Authentication Across Applications
Also, it is also possible to leverage some 3rd party centralized authentication service such as the Windows Live ID authentication for sharing authentication info acrossed multiple site/web applications.
#Windows Live ID Web Authentication SDK