The flip-side to "pretty" code is good documentation. There are too few programmers out there who can both write elegant code and explain what it does just as elegantly. I have his "The Inform Designer's Manual, Fourth Edition" sitting on my shelf right now, and I can honestly say that it is the only manual that I have had the pleasure of describing as a "page turner".
The code itself is equally impressive. Satiated with (helpful) comments, compiles on virtually any platform, easy to read, follow, and modify as you see fit. There's also a separate "Technical Manual" for those who are interested in modifying the original code.
Let's see... Let's not forget nethack. They thought of everything.
And then there are always those little tidbits of code like this inverse sqrt function that fall into the it just works category. Code that mere mortals such as myself can hardly comprehend - let alone write!
http://www.math.purdue.edu/~clomont/Math/Papers/2
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