Main Registration Modules
Email Registration
LoginToboggan
Lazy Registration :( maintainer?
User name generation
RealName
Auto username
OpenID (coming soon! Chris Messina wills it.)
OpenID Registration
OpenID Provider
OAuth
OAuth Integration
http://drupal.org/project/oauth_common
http://drupal.org/project/services_oauth
http://drupal.org/project/oauth_services
Invitations /Restrictions
Invite
Request Invitation (D5)
Registration Code (D5)
Registration Role (D5) (D6.dev)
Registration Modification for Roles (D5)
Pick from avatars on registration / or whenever (neat!)
Avatar Selection