Hi Christof.
The customized installation running the openacs.org website is available via CVS by issuing:
cvs -z3 -d:pserver:anonymous@openacs.org:/cvsroot co openacs.org-dev
Also browsable here:
http://cvs.openacs.org/cvs/openacs.org-dev/
Here is some discussion on the setup: https://openacs.org/forums/message-view?message_id=84297
There is a new package development tutorial available: https://openacs.org/doc/openacs-4-6-2/tutorial.html
I don't know of any central place on openacs.org for sharing of (customized) packages, except the contrib directory in CVS. Such a place would be cool. Something like the Lotus Sandbox maybe?