Does your site attempt to bring together a unique group of people to collaborate and contribute to your subject matter? The internet is a great place to connect people of similar interests, or even people with differing views who need a forum to deliver their opinion. There are a number of community-centric open source packages available:
Elgg
Elgg is an open source social engine which powers all kinds of social environments.
Mahara
An utility to manage your online portfolios. Also includes a resume builder.
LovdByLess
An emphasis on community involvement and friend activity, LovdByLess is a great example of an open source application running on Ruby on Rails.
TypePad Motion
From the makers of the popular open source blogging software Moveable Type, comes the community-based microblogging software TypePad Motion.