Please note that all contributions refer to drupal 6.x series.

• Canonical URL
  Module adds support for canonical URLs - concept adopted by Google, Microsoft and Yahoo.
  Drupal.org project page is http://drupal.org/project/canonical_url
• LJXP - LiveJournal Cross Poster
  This module does exactly what it's name says - posts drupal content to LiveJournal or other site using LJ engine.
  Drupal.org project page is http://drupal.org/project/ljxp
• OpenID autoregistration
  Intention to include OpenID to drupal 6 core modules was great, but, unfortunately, we don't leave in the perfect world.
  If you're used to sites encouraging you to log in using OpenID, and it works - then these sites are definitely not based on drupal.
  While openid module supports simple registration (SREG) and tries to fill user registration form with SREG data, great number of OpenID providers do not supply user email (sounds reasonable, ah?). In addition, user name taken from SREG often contains characters that fail drupal username validation. So, instead of logging in, user gets confused with series of error messages telling him he didn't enter email or his username contains illegal characters.
  Bottom line - if you want your site to silently accept OpenID logins - use this module :)
  Drupal.org project page is http://drupal.org/project/openid_autoreg
• taxonomy_theme
  taxonomy_theme module allows you to change the theme of a given node
  based on the taxonomy term, vocabulary or nodetype of that node. You can
  also theme your forums and map themes to Drupal pathes or path aliases
  directly.
  This is non-official fork of Thilo Wawrzik's module (port from drupal 5 version).
  Author suggests switching to another module if you use D6, as an alternative, you can use my module.