ChessDB - ChessDB - a free Chess database for Mac OS X, Windows, Linux, and UNIX - like ChessBase, but better
DecodeChess - AI chess tutor and analysis
Chess Tempo Database - Chess Tempo Database gives you a library of more than 2 million searchable chess games.
Chess Insight - Chess Insight is a chess tutor that helps you with a number of analytical powers for judging every move in chess.
Scid vs. PC - Chess database and playing program.
Lichess - The complete chess experience, play and compete in tournaments with friends others around the world.