Also free, if you are the kind to read language specifications:
R5RS: http://www.schemers.org/Documents/Standards/R5RS/r5rs.pdf R6RS: http://www.r6rs.org/final/r6rs.pdf R7RS: http://trac.sacrideo.us/wg/raw-attachment/wiki/WikiStart/r7rs.pdf Free implementations I can recommend: Guile (official GNU extension language!) Racket (comes with an IDE, good for both learning and developing) Chicken (compiles to C, relatively fast) Gauche (good R7RS support)
