The Half Race Handbook

A modular system for creating unique half-race characters

From the introduction:

In a living world, there are bound to be more half-races than simply half-elves and orcs. This supplement aims to create a fully modular race creation system, intended for use in creating characters of mixed races. Racial features have been deconstructed into “major” and “minor” features that can be combined to make more diverse characters. Not every racial feature is represented, as only features likely to be passed down genetically have been included. If desired, weapon and armor proficiencies usually found as racial features can be gained by taking them as feats or including them in a custom background.