In a Unix computer when I log in and open an application it loads my settings, my documents, my music, etc. Windows provides similar functionality, I believe. When I install an application, it doesn't ask me who I am, it just fetches it from somewhere, be it environment variables, or registry, or some other OS API. However, when you sign up for an online service or an application it asks who you are and you cannot say to fetch your data from another web site you have already registered with.
There are some services to provide this. As far as I remember MS tried to implement something like it. Facebook, provided a common framework for applications, open social is another alternative for Facebook's framework. Each comes with their own pros and cons. However the real problem is there is no unified way for it.
Now, there is a somewhat similar standard created to provide a similar functionality,
OpenID. It does not handle all the issues, but surely a starting point. There is a lot of technical definition on the OpenID web site. Here I will show a sample scenario, in which I use my blogger account as my OpenID identity and connect my sourceforge account to it.
Log in to the sourceforge.net site.
Go Account Options -> Manage My OpenIDs -> Add an OpenID -> Enter your blogger url -> Click Log In -> Enter Your Password to Blogger if it asks -> Say "Yes, Always"
To test it: Click log out. -> Click Log In at sf.net page -> Enter your openid url -> There will be a blogger account login screen if you are not signed in to your blogger account. Enter your mail and password -> Well, that's it. You have an online identity.
Have fun.