The Aolserver web server is one of the most readable C programs I've ever seen. Each function definition is preceded by documentation of its purpose and side effects. Unfortunately the modules tend not to be quite as clean. The development community is dwindling but exceedingly polite and helpful.

The one gripe I have is that the tuning parameters are not well documented (especially post 4.5.0). You have to study the source in order to dig out all the parameters you need to tweak in order to really make the server sing.