AI (Artificial Intelligence) applications, tools, frameworks, etc.
Common Lisp also happens to be an excellent language for AI.
The CMU Artificial Intelligence Repository contains a wealth of both Lisp and AI (as well as Scheme and Prolog) related documents and code.
The Artificial Intelligence Algorithms Wiki was a new AI resource, but it is currently offline. The Artificial Intelligence Wiki may still be available.
If you use GNU/Linux (and perhaps even if you don't), see also the
GNU/Linux AI & Alife HOWTO
Theorem Provers, previously listed under AI, have been moved to their own subtopic. Other subtopics include Knowledge Representation Systems, Natural Language Processing, Distributed AI...
Building Problem Solvers describes how to build reasoning systems, using Common Lisp source code available at this web site. The code includes a variety of rule engines, truth maintenance systems, and constraint systems.