So Pe
SOPE
An OpenSource alternative to WebObjects which contains various extensions to WebObjects in the spirit of Zope, including support for XML-RPC or WebDav.
Widely known as the application server basis for OpenGroupware, a huge OpenSource web application consisting of about 300 reusable WebComponents and various applications based on the SOPE server. 7-8 years into development, SOPE is extremely mature and well optimized.
SOPE:X, a part of SOPE, has advanced support for the MacOsx platform and development tools (like WebObjectsBuilder and ExCode).
See also: GnuStepWeb