Once upon a time there was a Content Recommendation Engine that showed a lot of promise. http://drupal.org/project/cre It never got beyond Drupal 5.
The concept, to provide recommendations of content based not just on the content but on the user's own usage metrics, is in much use on the web, but as far as I know there is no viable solution currently in Drupal.
A Content Recommendation Engine update to Drupal 7 would be awesome, and something that could be used by almost any Drupal site that has active members and readers.
I would be happy to volunteer as a mentor on user behavior algorithms and user interface design. A D7 code mentor would be needed as well, imho.