Rick  Strahl has just posted a really comprehensive post on Integrating OpenID in an ASP.NET MVC Application using DotNetOpenAuth.

OpenId is a single sign on (SSO) scheme. The idea here is that you use your login for one site all over the place. SSO isn't really anything new, however, from looking at OpenId the list of providers is fairly comprehensive and with absolutly everyone having loads of accounts nowadays I can see OpenId really taking off.