OpenACS is a web application platform that is often overlooked, based on AOLServer and PostgreSQL.
OpenACS also comes with a whole set of ready-made modules - true to its name as Ars Digita Community System, it offers a whole stable of ready-made applications. A complete CMS is also included. And all of this is partly several years old - from times when other CMS projects were still dreaming of being conceived.
The AOLServer is a multithreaded web server that uses TCL as a scripting language, just as Apache is often combined with PHP. However, the AOLServer is very fast and surprisingly resource-efficient. The thing is called that for a good reason: larger parts of AOL run on the server and it originates from there. It's still worth something.
All in all, an exciting old-timer with interesting features and - due to its age - quite good documentation. But also some warts and edges that may seem a bit strange nowadays. One must keep in mind that when ACS was developed, the definition of CMS was only in its infancy.