Some open source expert systems / expert systems tools (tools you can use to write Expert Systems) include
- C Language Integrated Production System (CLIPS), CLIPS is an enviroment used to make rule or o bject based expert systems
- Python Knowledge Engine Pyke, Pyke allows you to use Logic Programming to make expert systems in Python
- OpenExpert PHP Expert System Tool mainly focused on application for Legal Expert Systems.
- d3web is Java Knowledge Base System that uses XML
- jColibri Reference Platform for Case Base Reasoning Programs in Java
- DTRules Decision Table based rules engine in Java
- drools is a well supported Java based rule-processing engine
- EulerEuler is an inference engine supporting logic based proofs.
- Infosapient Java Business Rules Engine
- Jena Jena is a Java framework which includes a rule-based inference engine, a ontology API f and a query engine
- JEOps JEOPS adds forward chaining, first-order production rules to Java in order facilitate expert systems development using declarative programming
- JLisa A CLips like rule engine with a Common Lisp interface in Java
- mandarax A derivation rule compiler for Java
- ofBiz Java based Business Rules Engine
- OpenCycOpenCyc is the open source version of the Cyc technology, the world's largest and most complete general knowledge base and commonsense reasoning engine.
- DEX DEX is an interactive computer program for the development of qualitative multi-attribute decision models and the evaluation of options
Additional relevant resources can be found in the list here.
In terms of recommendations at which Expert Systems to look at for learning purposes, I would recommend OpenCyC. There is very intereting Google Tech Talk Computers vs Common Sense about the Cyc technology.
Without additional information and clarification it is difficult to make further recommendations.