OpenID is a great implementation of a great idea. Vendor agnostic single sign-on with multiple persona may finally happen. For a good intro, watch Django's Simon Wilson's presentation here:
I uses 37signals basecamp. This week I started with someone that also uses basecamp. So now I am a "owner" and a "client" so to speak. That means two different user IDs at two different basecamp URL. While I have been complaining about 37signals's apparent lack of progress in new features in the products, they have been secretly working on openID. Now you can use openID to log into their services, and everything is available with one URL! See www.37signals.com/openbar for more info.