ACL2 is a mathematical logic, programming language, and mechanical theorem prover based on the applicative subset of Common Lisp. It is an "industrial-strength" version of the NQTHM or Boyer/Moore theorem prover, and has been used for the formal verification of commercial microprocessors, the Java Virtual Machine, interesting algorithms, and so forth.
Fract is a Web-based fractal zoomer for the Mandelbrot set. It has no advanced features, for the goal is to keep the code easy to read for someone interested in fractals. The package also includes an offline renderer that you can use to generate high resolution images of your favorite spots. You can also easily play with the color map.
Charlemagne is a versatile genetic programming application. It includes a commandline client and an interactive console mode. It is written in Python and Lisp, and is user extensible to some degree in both languages. It features built-in input-output mapping support and provides the ability to define complex fitness calculations in Lisp or Python.