Microsoft .NET Passport is a user authentication system that provides a VirtualIdentity. It provides single-sign-on capabilities: once a user logs in, that user can access all participating web sites without the need for additional logins.
The prefix of DotNet is a misnomer as its use does not require MicrosoftDotNet. It has been around before DotNet days and got attached to the DotNetbrand because of the expanded role it was expected to take up around that time.
Ebay has been a big supporter of the MicrosoftWay, but in Aug04 MS reduced the scope of DotNetPassport. In Jan05 Ebay formalised its withdrawl from this authentication service.
eBay revokes Microsoft's Passport at http://news.com.com/eBay+officially+revokes+Microsofts+Passport/2100-7345_3-5542438.html
settlement of misrepresentation at http://www.ftc.gov/opa/2002/08/microsoft.htm
MicrosoftSecurity flaws found (2003 article) at http://www.eweek.com/article2/0,1759,1660033,00.asp