Updated Sept 19 2008:

From the article "Using Prolog as the fundament for applications on the semantic web":

March 28 2009: added links, few edits. revisiting subject in general

There are some Javascript oldies but goodies to be found on the Web.

There was an active Javascript inference engine at CNLP ( Syracuse Univeristy ) at some point, but it may be moribund. From the site:

... building business rule applications or rule-based systems using the Java programming language.

* Promote rule-based programming in Java in general.

* Provide a vendor neutral portal for finding and sharing information.

* Inform and educate implementors and users of the "Java Rule Engine API" (JSR-94).

* Discuss future evolutions of the Java Rule Engine API.

Java, Eclipse plugins, server supporting WSDL.

A very big, enterprise level Java RE environment ...

Excel + Java + Eclipse

A combination of Excel, Eclipse, and Open Source Tools, backed by OpenRules support, forms a practical framework for full-scale Business Rules Management and rules-based Web Application Development. OpenRules embodies an ingenious software architecture that maximizes customizability and is open to any integration solution. Extensive reuse of proven, commonly available software results in a framework that matches or exceeds similar commercial systems in speed, compactness, and ease of use.

http://openrules.com/overview.htm