Google AppEngine and OpenID
I do have real security and privacy concerns when one account will log you into many services. I keep an encrypted text file on my laptop with all my web and server logins and use different random passwords for different services. Having many different logins does make online work more secure but this has to be balanced against saved time and greater convenience. At the end of the day, I feel more comfortable keeping logins for online banking, health record access, etc. separate, while enjoying single login experiences with services that are less sensitive.