With 5 levels of difficulty and a behavior modeled to make human mistakes, this is a very fun game to play.
SparkChess is an excellent way to get better at chess - learn the proper rules (including the elusive en-passant), practice openings, test strategies, use the board editor to recreate famous positions with FEN strings, replay famous games, import/export PGN games and databases (with comments and annotations) and let the computer help you. SparkChess is a free online chess game that allows you to practice chess against the computer and to engage in multiplayer online challenges, or to just watch others play.
