The SKYRiX Object Publishing Environment is a free variant of a web application framework based on the WebObjects model. Specifically, considering that WebObjects is now migrating from Objective-C to Java, SOPE is very interesting - as it is still entirely Objective-C.
Moreover, it runs not only on OS X, but also on Linux. And of course, I like that it is based on PostgreSQL and not on this glorified index card box ...
Would be a good reason to refresh my somewhat rusty Objective-C skills. However, the documentation is still quite sparse - but you should be able to refer to the WebObjects documentation for many areas.
By the way, for Debian Sarge there is a repository with ready-made packages for SOPE. It is not yet on their homepage, only on the Freshmeat project page for SOPE.
It is, by the way, the basis for OpenGroupware.org - a groupware construction kit (according to the developers themselves). And there is a Live-CD with which you can test the whole monster without having to install much.
Can someone explain to me why I have never seen this project before? Do I have tomatoes on my eyes? Strange ...
A warning to those who want to get started (I just compiled, installed and tested everything): describing the documentation for SOPE itself as non-existent would be flattering.
Tip: a WebObjects application is so to speak a small web server in itself - you simply start it and attach Apache to it using the mod_ngobjweb module to this small mini-server, and then you can access the elements of the application. It is not immediately obvious for someone who does not know WebObjects ...