With the chess fever spreading because of the ongoing WCC 2013 between Anand and Carlsen, here is an industry standard open source chess engine you can develop your chess skills with: Stockfish. It has all the features of any top-notch commercial chess engine and is second in strength only to Houdini (commercial) right now. It is multiplatform and easy to install on any popular desktop or mobile OS. The GUI application on Android is Droidfish. Readily available in synaptic on Ubuntu. You can use the Stockfish engine with PyChess on Ubuntu. You can also use it with eboard by converting the UCI protocol Stockfish uses to xboard. Check this thread for details.