ArsDigita Community System (ACS at http://software.arsdigita.com) is a Web Community System.
It is GPL'ed, which in short means it is licensed to protects all user and programmer rights and promote world wide development of ACS.
From a 10,000 feet view the ACS is not unlike a distribution of your favorite OSS web script modules--packaged to cooperate with each other. The difference between this one tarball and the chaotic nature of Freshmeat.net is the central development and administration web community at http://www.photo.net/wtr (web/db). With the combined expertise and assistance of ACS users and feedback from all of the world--the Ars Digita company is able to bring multiple folds and facets of benefits with each mating of distinct and loosely-coupled features. It's more than a distro of all those major web components you see in the web scripts section of Freshmeat.net glue into one script distro. But a federation pushing for cooperation.
ACS uses AOLserver, Tcl, Oracle, mainstream Un*xes and Linux.
If Oracle is too much big business for the average ACS admin to handle--a 100% OSS developed is in the works. ACS/Pg will provide you with a modern port of ACS to the PostgreSQL DB on x86 Linux platform.
ACS/Pg is at http://acspg.benadida.com/
Update: /ACS/Pg is now called OpenACS. You can find it at http://www.openacs.org/
ACS was first started by Philip Greenspun and friends way back when the web was young. The project spawn a company off-shoot call ArsDigita, dedicated to changing the world in a good way and getting paid for it too. Philip offers many helpful free database and web services to aspiring web masters. You can learn more at his homepage (http://photo.net/philg/) and at his own company (http://www.arsdigita.com)
A humorous, friendly and insightful readme into the ecology of ACS is provided by this book: Philip and Alex's Guide to Web Publishing
ACS is one of the few web projects in the world designed for the users by the users (see Everything2, Slashdot, IMP, ACS). This means just about all design decisions and implementation trade-offs were made in the real-world interest of everyday community of users of sites made with these web toolkits.