Go! is an open source groupware package. It began life as a simple clone of Yahoo groups and grew from there. I think I worked on it for about a year, and by the time I stopped it had quite a few features.
Go! is group based, meaning each group has a homepage, forum, task manager, event manager, gallery, knowledge base, file storage area, calendar etc etc. Of course these can be turned off and on to suit the group requirements. Email gateways are supported, so Go! acts as a mailing list sending and recieving emails from users. RSS feeds are also supported.
Go! provides a rich text editor so browsers that support it can edit the pages/messages using a familiar interface. It also features HTML to text convertion (and back the other way) so you can clients can view the data in whatever format is more useful
Go! uses a method similar to CVS to store the difference between messages when they are edited, full history is available for each node. Finally it support skins, to change the appearance
Go! in written in Perl using a templating engine called Mason. It is backended using MySQL and (optionally) LDAP for user authentication. More screenshots are here, or you can visit the sourceforge page.