an updated and heavily commented version of norvig's lispy. it should be read in conjunction with this and this and this.
as for why this matters: "scheme demonstrates that a very small number of rules for forming expressions, with no restrictions on how they are composed, suffice to form a practical and efficient programming language."
to use, do python lispy.py.
>>> python lispy.py
>>> (* (+ 2 3) 3) # returns 15