I have also been involved in the design and implementation of the R++ rule extension to C++. R++ adds path-based rules to C++ in a manner that fits nicely into the object-oriented nature of C++. For more information on R++, see my page on R++.
Unfortunately, due to legal issues between AT&T and Lucent, R++ cannot currently be distributed. This is unlikely to change.
I have been the co-chair of the DARPA Knowledge Representation System Specification (KRSS) group of the Knowledge Sharing Initiative. The group produced a specification (LaTex2e source) for Description Logic systems.