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Lisp Machines were used by NASA to check Space Shuttle starts. From a Symbolics press release from 1990:
"Recently the National Aeronautical and Space Administration (NASA) used Symbolics' high-definition technology to analyze HDTV video images of the Discovery launch in real-time. This high-definition system enabled NASA engineers to get an instant replay of critical launch systems. The engineers were able to enhance and enlarge high-resolution images of the lift-off in order to analyze the condition of and spot potential problems with space shuttle tiles."
They used a bunch of Lisp Machines with special image processing boards.
Probably, not the most impressive, but really worth mentioning, considering, that it's mostly a one-man effort: http://piano.aero/
It's hard to say which Lisp software is exactly most impressive, but I would add Symbolics Genera and related software to the list of achievements worth mentioning. Also don't forget SLIME.