What is the most impressive Lisp application? [closed]

Question

I know that this is subjective and all, but still, can you provide some list of serious applications that were written in Lisp (perhaps along with what Lisp it is)?

Answer 1

Emacs.

At least parts of it are written in Emacs Lisp.

Answer 2

ITA Software's airline-fare search service. See here to understand why this is impressive.

Answer 3

LISP itself! :)