Two projects you may want to review for ideas are Redis and toybox.

Redis comes to mind because it started out as largely a single file of code that has since been split and organized into multiple files. The code is quite approachable; you'll likely understand how most of it works after a day of causal browsing. http://redis.io

Toybox comes to mind because it's insanely modular, and aggressive about code re-use. The logic can feel a bit dense at times, but he's going for size and speed. I'm a big fan of Rob's efforts. http://landley.net/code/toybox/