Apache-mod_helloworld is a project mainly written in C, it's free.
An hello world module for Apache 2.2
This module code is taken from the companion site to the The Apache Modules Book and corresponds to the example on Chapter 5.
Although the output differs from the Nginx hello world module a comparison between both modules is appropriate. The Nginx module is more complex, but gives you higher performance and a finer control of the way the module operates.
The code is licensed under the GPL v2.