After much hard work led by James Walker, OpenID 2.0 support is now in Drupal 6 core!
OpenID is a decentralized, secure single sign-on system. It allows you to create a login at a site you frequent, and use that same login on any other site that supports OpenID. Drupal 5.x and below have supported this with the built-in Drupal module, but OpenID is an open standard, better supported, and more secure.
"Let this be the day where we help revolutionize the online society, and the way websites and web services interoperate. Or something." - Dries