I had this dumb idea of actually evaluating LISP in a forth inspired way. It would be a stack of subroutine/accumulator pairs. so given

    (+ 1 2 3)

"(" would pop an pair on a stack, + would set its subroutine, and 1 2 3 would be fed to the subroutine and accumulate a value. when ) is encountered, the data is fed into the subroutine/accumulator pair lower on the stack, so you could still have nesting.

Horribly inefficient, but I thought the idea was neat. I'm sure I'm not the first to think of it though.