A while ago I patched Michael Garland's qslim to compile on modern Mac OS X. I've finally wrangled in the instructions and modified files to a github repo. There are also wrappers for this tool in my gptoolbox. It'd be cool to have a proper C++ API to this tool, but that's future work.
Qslim is one of the seminal mesh decimation/edge-collapse/simplification techniques. It's fast, simple and predictable. It's a good starting place if you're trying to simplify your mesh, especially in an automated way.