Code
gray-matter
Gray Matter is a chess engine; it can play a game of chess. It isn't very strong but is a good learning and teaching tool. Its authors aim to write readable, understandable, and hackable code. It's written in C++ and employs good object-oriented practices. It uses some state-of-the-art techniques such as an MTD(f) move search and rotated BitBoards.
Gray Matter doesn't have its own interface. However, it conforms to the Chess Engine Communication Protocol and runs under XBoard.
Maintaining a provisional rating of 1200 while weighing in at a svelte 3000 lines of code, Gray Matter offers a nice bang to buck ratio. That's probably why its adoring fans (both of them!) affectionately refer to it as The Little Engine that Could.