Facebook is popular for that, but personally I use Twitter, Google and Yahoo (via their openid services) for login to several sites, including this one. Faceboo

Facebook is popular for that, but personally I use Twitter, Google and Yahoo (via their openid services) for login to several sites, including this one. Facebook has by no means won the contest to provide a 'single login'. I don't think anybody is going to. Facebook is my last choice for signing into a site, as they give the site access to more data then is needed.