The neatest feature that I have ever made in a game is my own Quake-style console. It is actually used in a published game. Well, the version of it written in C is used in a published game. That was back in 2001. Now I have resurrected it for the latest project I am working on at Krome, with enhancements that make it suitable as a basic scripting language. It is also used in my Donk engine.