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.
Great news. OpenID is the future. Just like RSS and microformats, I hope that OpenID will be supported by everyone.
(Via Drupal.)