Rodent II 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.
Rodent's strength is probably about 2870 Elo on the CCRL scale. Previous version hovered slightly below the 50th place on CCRL and CEGT rating lists. You can conclude that Rodent II is a decent engine, albeit far from the top dogs.
Rodent II is just a chess engine, so in order to enjoy it you will need a graphical user interface (GUI). Some of possible choices are: Winboard, Arena, TarraschGUI, or recently discontinued ChessGUI. If You are using another GUI, You should be fine as long as it understands UCI protocol (which is usually the case, since top engines like Stockfish and Komodo use it).
Rodent II is a derivative of much weaker, yet very cleanly written engine Sungorus by Pablo Vazquez. Its concise and logical code served as a base to add further enhancements. Throughout the developement cycle, a lot of knowledge from other open-source programs has been added. These inspirations include:
Andscacs – penalty for bishops that cannot reach enemy half of the board
Crafty – material imbalance table
DiscoCheck – evaluation of hanging pieces.
Fruit (via Toga log user manual) – many evaluation weights, including piece square tables. Fruit values served as reasonable first guesses and some of them were subsequently retuned.
Gambit Fruit – king tropism weights.
Hakapeliitta – basis for pst tables (Rodent's are symmetric).
Senpai – implementation of aspiration windows and late move pruning
Stockfish – late move reduction depths, the way of initializing king safety table.
Toga II 3.0 – razoring implementation and forwardness eval.