Updated Oct 3 2008:

C++ engines are often neglected for web-based inferencing applications, but compilation is possible on non-shared hosting ...

The granddaddy of the clan is CLIPS. It has a complete and well-documented rule language that I have found to be suitable for technically-oriented people in engineering applications, but to be impossible for ordinary business folks to express their business rules. [ A more modern version is RuleML, in its various forms ].

CLIPS is a prime examples of how collaborative open source applications can protect a company's software investment. The project has trundled along in various incarnations since the late 1980s and nearly blinked out of existence a few times. If it had been the commercial product of a big corporation, it would have been axed a dozen years ago. Like so many other open source projects, the CLIPS project may not be going anywhere in a hurry, but it isn't going away either.