Cool Single Sign On Service

Just wanted to tell people about a new service I found that offers free single sign on services and allows registration using facebook, google, etc Also has a plugin in wp called RPX.

I'm sharing because I've looking for something like this for a while now.

If you are using multi-domain (i think) or sub directories on MU you don't need this. If you have separate installations of WP or other CMS' like I do this may come in handy.