
Try OpenACS. It is an open source (GNU GPL) toolkit for building scalable, community-oriented web applications. Oracle and PostgreSQL are the supported back-end databases and the programming language is TCL/TK. The developer community is almost 9000 strong which is very helpful. The toolkit is very mature and is useful to build community-based websites quickly and has a host of advantages over other similar systems.
More information about the toolkit can be found at the OpenACS website "What is OpenACS" section and on my personal website - About OpenACS. A quick comparision of the CMS applications available can be found at www.cmsmatrix.org (Link to Google's cache as the site doesn't actually have the comparision feature anymore now! Maybe you need to register?!)
Those who can, do; those who can't, write. Those who can't write work for the Bell Labs Record.