Ask HN: What are some widely used programs that are written in Lisp?

I'm trying to get my hands dirty in using LISP for developing practical programs, I'd like to study the source code of some actual program that are written in the language.

If I were to study C++, I'd grab the Quake 2 source code.

I've heard the language is mind-blowingly-amazing, so I want to read some programs that are also amazing.