While I normally work on Drupal projects for paying clients this was something I've been working on for some time now with a friend, partly as a portfolio building project but also because building an active social networking site is really interesting.
Cosplay Island is a social networking and community site for anime, manga, sci-fi fans in the UK who like to attend events such as conventions dressed as their favourite characters.
CarChannel is a site dedicated to cars (only in Dutch and French) with a lot of video content and some community features: you can create your own 'channels' and gather youtube/video/whatever videos around one subject/model. There is of course also rating and comments. Available at http://www.carchannel.be
I have to say, I have been working with Drupal 5 now and really like it. In a matter of less than 4 days, I was able to create a pretty full featured community site for Chevy Van owners. My real goal was to see if I can do it all Drupal based without code modification, which actually worked.
Now, the site is still rough from my own user experience standpoint, but I think with refinements, it will be evolutionary.