OpenACS (Open Architecture Community System) is an advanced toolkit for building scalable, community-oriented web applications. If you're thinking of building an enterprise-level web application, OpenACS is a solid, scalable framework for building dynamic content driven sites.
OpenACS is a collection of pre-built applications and services that you can use to build your web site/application. Through a modular architecture, OpenACS has packages for user/groups management, content management, e-commerce, news, FAQs, calendar, forums, bug tracking, full-text searching, and much more.
OpenACS relies on AOLserver, the free, multithreaded, scalable, Tcl-enabled, web/application server used by America Online for most of its web sites, and a true ACID-compliant Relational Database Management System (RDBMS). Currently OpenACS supports PostgreSQL, an open source RDBMS, and Oracle and is easily extensible to other databases which support a comparable feature set.
The OpenACS toolkit is derived from the ArsDigita Community System (ACS). ArsDigita (now part of Red Hat, Inc.) kindly made their work available under the GPL, making all of this possible.
The OpenACS project was born when Don Baccus, Ben Adida, and others decided to port ACS from Oracle to PostgreSQL, thus making it a fully open-source solution. With OpenACS 4, Oracle and PostgreSQL support were combined in one code base and with OpenACS 5, support for internationalization and localization has been added.
A vibrant and productive community has sprung up around the OpenACS software and there are many volunteer contributors as well as a commercial companies able to provide support, hosting, and custom development. Many of the production users are actively funding and contributing work back to the project. Formal, consensus driven governance has been established (with semi-annual elections) which ensures the project serves the needs of it's constituents.