[sparkie]

Another small lisp implementation: http://piumarta.com/software/lysp/

Also, another, more interesting Lisp by the same author: (http://piumarta.com/software/maru/). Maru is basically a lisp where some of the core functions like eval and apply are extensible from user code. There's basically a global map of types to evaluators and applicators, with some functions for you to register your own types and get the evaluation behavior you want.