Login once at site A and then automatically be logged on to sites B, C, and D as well? You're not dreaming: you're using Janrain Single Sign-On.
SSO segments, site protocols, and other things you need to know.
Single Sign-On for the Registration UI
Configuring Single Sign-On
with the Registration UI.
Single Sign-On for the Registration APIs
Configuring Single Sign-On without the Registration UI.