Mathematical/logical

• Function space illustration -- one attempt to visualize the set of functions from an N-element set to itself
• Binary branching illustrations
• Constructive theorem prover for propositional calculus [tar file compressed using gzip] -- given a simple type, compute lambda-expressions that have that type
• Solid modeling system in Scheme, vaguely inspired by algebraic topology and Bruce Donald's EIGHT modeler
• Representing mathematical knowledge in Ontic (with David McAllester)

Miscellaneous

• ERA: Environment for Reflective Agents, with Kent Pitman and Sankar Virdhagriswaran, at Crystaliz, Inc.
• A4: Change, configuration, and inconsistency management for globally distributed, mobile, potentially replicated objects. At Crystaliz, Inc. There is a brief paper on it (MS Word version 7 format).
• Handy utilities for generating plots and figures (in Scheme, generating postscript output). I dislike most illustration/drawing programs (in fact, most programs, period) because they're not programmable. (Example)