P4wn
a quite small open source chess engine by Douglas Bagnall, written in JavaScript, released under CC0 or public domain terms, and hosted on GitHub [2]. It runs in web browsers and as standalone JavaScript engine (though less usefully there, without any UI). The first version was released in 2002 as an entry in a 5k web page competition, and its source code was incomprehensibly succinct. It used a sort of principal variation search and a piece-square table oracle. In 2012 it was rewritten for clarity and for performance in modern JavaScript engines. It now uses a plain alpha-beta search and more sophisticated piece-square tables. p4wn plays by all the rules, though it never claims a draw, just makes the offer and tries to avoid draws if it thinks it is winning. It likes to make rash sacrifices when ahead.