The Wonders of Winged Edge
A few months ago I started working on a commercial game for Binary Cocoa called BOCO. BOCO is a simple 2D puzzle/board game designed to be played with friends. One of the main problems I was faced with when working on BOCO was for each face on the gameboard to be aware of its neighbours. I wrote a blog post about solving this problem over on Binary Cocoa's dev blog. You can check that post out here, it's a good read.
TL;DR: Winged Edge is super useful and I wrote a pure Lua library for it.