I saw Matt Might upvoted another answer, but I will still post this 
wonder found on his blog: a 7-line interpreter for the Lambda Calculus 
in Scheme.

    (define (eval e env) (cond
      ((symbol? e)       (cadr (assq e env)))
      ((eq? (car e) 'λ)  (cons e env))
      (else              (apply (eval (car e) env) (eval (cadr e) env)))))
    (define (apply f x)
      (eval (cddr (car f)) (cons (list (cadr (car f)) x) (cdr f))))
    (display (eval (read) '())) (newline)