Dan Lennox @ emspace has gone to a lot of trouble reviewing drupal's contrib themes. This is really useful for anyone seeking a contrib theme for their client's site.
Chris Messina's (aka Factory Joe) review of drupal 6 is a big contribution to drupal usability. I have highlighted about 50 actionable issues that have been raised and/or discussed through this review. Most of them are trivial changes for drupal 6, but are key usability improvements. Many are simply improving the help text. Some are larger tasks that require more work and changes and should probably wait for drupal 7.
This is an interesting and probably also useful view of all contrib modules; http://www.kentbye.com/files/drupal_modules_all.html
and images on Flickr.
KentBye is the creator of the visualise backtrace module, which also kicks arse! Go Kent!
My largest code contribution to Drupal core just got committed! :) At last node_teaser() can take a rest -- at least for a while anyway. It looks like there'll be another round of work on it for Drupal 7).