Open Atrium: customizable, open-source team portal
It looks like the folks behind Open Atrium are pushing to create a feature-developer community, where people can build and share features for everyone to take advantage of. If you've always felt like every collaboration option out there was missing one feature or another, Atrium might be able to scratch that itch. My favorite basic feature is the iGoogle-like dashboard, which looks like the glue that holds all the other parts together. On the technical side of things, the core of Atrium is Drupal, and also uses PHP, Apache and MySQL.