Examples of excellent Common Lisp code?

CL-PPCRE is often cited as a good example, for good reason. Actually, probably any of Edi Weitz's libraries will make good reading, but CL-PPCRE is particularly clever and it's a useful and impressive library. Beyond that a lot of CL implementations are written mostly in CL. It can be pretty productive to pick some part of CL that's usually implemented in CL and compare how different implementations handle it. In particular, some of the best examples of large useful macro systems are implementations of things in the standard. Loop is an interesting read, or if you're really ambitious you could compare a few implementations of CLOS.

If there's some area of computing you are particularly interested in it might be worth mentioning that, so people can tailor recommendations to that.

It's another book, so it may not be precisely what you're looking for, but Peter Norvig's Paradigms in Artificial Intelligence Programming contains a lot of well-written, smallish Common Lisp programs. It's not perfectly natural code, especially in the the first few chapters, because, like code in Practical Common Lisp, it focuses on teaching you how to program in CL, but it's still very much worth a read. It also contains some excellent examples of ways you can build other languages on top of Common Lisp, and it has some valuable advice on how to improve the performance of CL programs.

The other recommendations (PAIP and CL-PPCRE) are excellent. I would also suggest becoming acquainted with Alexandria's code and also taking a look at GBBopen.