Laws of Logins

Privacy issues aside, when it comes to logins, I want convenience.

When using a web-page that provides a service, I want it to:

1. Allow me to try or taste the service without entering a username and password (or I won't it use it).
2. Never supply username and password if the service is once only (or I won't use it).
   
3. Allow me to login and use a nice administrative interface if it is a site that allows me to create content. For this, I not only accept having to supply login details; I want to be able to login.
   

Of course having to remember multiple logins for multiple sites (point 3) is a PITA. Can OpenID help with this?