The OpenID authentication protocol looks very promising. A number of efforts have already been attempted to provide a single sign-on service for various sites, but they have often failed because of political or technical reasons. OpenID looks different as it seems to have widespread, growing support.
OpenID is a distributed network of user accounts, where a user is identified by a unique URL. The URL in question is a real location on the Internet which points to a provider of OpenID accounts, e.g. MyOpenID.com (note: now dead, see below). When a user attempts to log into a site which supports OpenID they provide their unique URL: the site then forwards the request to that URL where the user must provide their password to log in; and they are then returned to the site requesting the authentication once they have logged in successfully.
It all sounds pretty promising so far, and I may look into adding OpenID support to Alpha for the version 1 release. More reading for those interested:
Some well-judged criticism:
"OpenID still open to abuse" - http://www.itweek.co.uk/itweek/comment/2184695/openid-open-abuse
Updated 2021 : note that the above post is out of date as it was originally published in 2007, but is left here for archival purposes. I have unlinked most of the external links above as they are now dead.