Object-orientation in C

Question

What would be a set of nifty preprocessor hacks (ANSI C89/ISO C90 compatible) which enable some kind of ugly (but usable) object-orientation in C?

I am familiar with

Answer 1

ffmpeg (a toolkit for video processing) is written in straight C (and assembly language), but using an object-oriented style. It's full of structs with function pointers. There are a set of factory functions that initialize the structs with the appropriate "method" pointers.