Pilo Visual Tools for Scheme (Pvts) is a basic interpreter with visual tools for Scheme (LISP). Written in Java, PVTS has an interpreter and three visualization modules: a global environment viewer, a function call viewer, and a data structure viewer.
An implementation of the language described in "Karel the Robot: A Gentle Introduction to The Art of Programming" by Richard Pattis et al. Both Java and C/Unix versions are available.