Apache's C API is quite extensive, and merits one or more talks of its own, but here are a few things we have found particularly useful. A big thanks to all the httpd developers!

• pools
  • A memory management system that simplifies life a lot for the module author!
• apreq
  • good system for dealing with user-supplied data - cookies, GET and POST variables, etc...
• apxs
  • Useful for building extensions - gives you the compile options you need to find and use the same build environment as Apache.