Rodent III is an open source chess engine by Pawel Koziol, licensed under GPL 3.0. It has been programmed for tunability. You can turn it into a crazy attacker, an old-fashioned positional player, or a sparring partner for Your kids. Its tuning options are both intuitive and relevant from the chess point of view, making it a perfect free replacement of commercial programs.
Home page: http://www.pkoziol.cal24.pl/rodent/rodent.htm
Download current development builds (master branch):