Deep Sjeng 1.x is a professional chess playing and analysis engine. It plays above grandmaster level (more than 200 ELO stronger than 'Sjeng Free'), supports multiprocessors, different personalities which can simulate human errors, multivariation analysis, access to internet chess servers, a database, a professional hand-tuned openings book and all the other goodies you could want.
It comes with the ChessPartner graphical interface and supports UCI, WinBoard I and WinBoard II, allowing it to be used in other interfaces such as ChessMaster, Fritz, WinBoard, Arena, ChessPartner, ChessBase, Chess Academy and more. The package includes a version of the engine (without interface) for Linux and Mac OS X.
Here is a screenshot.
Deep Sjeng 1.x has been superseded by Deep Sjeng 3.x