For those of us who actually ship code, I'd like to suggest khash[1], which is a pretty nice implementation of a hash table in C. It uses lot's of macros, so if you're looking for a brain teaser you will be satisfied as well :)
I don't think being implemented with macros is a plus. I've seen some friends use it and get stuck whenever it was failing because it's impossible to know how it's working. It's the same type of cleverness some people criticize in other comments. Yes, it may be complete and useful but it's opaque.
[1]: http://attractivechaos.github.io/klib/#Khash%3A%20generic%20...