non-Drupal software

I've just come across the fantastic software for social bookmarking for academics Connotea (see http://www.connotea.org) and started thinking about the many great projects out there which we could either learn from or integrate into Drupal. Of course, there are some obvious candidates such as Moodle, Elgg, Gallery2 that we all know about. But what about services such as Shelfari or LibraryThing? So I thought I'd start a thread where we could talk about what others are doing and how we can benefit from their hard work.