Open Ars Digita Community Server.
Based on the "old" version of Ars Digita, so it is in Tcl (the new one is in Java). Tries to make it easy to access an underlying database from anywhere in the code. Uses AOLServer. Has a "data model" with routines to deal with "objects" of data of various types (types are like OOP classes). Comes with lots of modules for common tasks (poll, message board, etc). The object data model supports privilages (i.e. some users don't have access to some things).
There is a Wiki module available for OpenACS at http://cvs.openacs.org/cvs/openacs-4/packages/wiki/
OpenACS is used to build some of the biggest online communities in the world including http://www.photo.net/ and http://greenpeace.org/
Also, there are two specialized community system derived from it:
- DotLrn (http://dotlrn.org/ is an e-learning platform used by MIT Sloan School of Management, University Heidelberg and others
- ProjectOpen (http://www.project-open.com/ is a mix between an ERP, a ProjectManagement system and an OnlineCommunity