Qmail: http://cr.yp.to/qmail.html

It feels well organized almost throughout, and it's a joy to experience the clarity of thought that went into it. In fact, most of djb's code has a similar feeling to it.

Arthur Whitney's code: http://www.nsl.com/papers/origins.htm

Being able to read this (and not merely decode it, but read it, in a manner similar to how you would read a book) will broaden your horizons like nothing else I've seen.