[NBJack]

A tower defense game whose core concept was from a chance reading of a Game Programming Gems book in the university library. Large numbers of bad guys in screen with solid frame rates, home built engine, and an AI that always attempts to thwart you by trying new paths into your base to defeat you (but also runs in a separated thread that pathfinds without hurting your framerate on a dual core or better system). Did I mention I started it on the old XNA library from Microsoft back when they offered the $99 license to make games?

Hasn't made a cent yet, but it has been a LOT of fun to build from scratch and tinker with over the years.