DecodeChess - AI chess tutor and analysis
ChessDB - ChessDB - a free Chess database for Mac OS X, Windows, Linux, and UNIX - like ChessBase, but better
Chess Tempo Database - Chess Tempo Database gives you a library of more than 2 million searchable chess games.
ChessPad - ChessPad is a feature-rich chess game viewer and PGN file editor available for Windows.
Lichess - The complete chess experience, play and compete in tournaments with friends others around the world.
Scid vs. PC - Chess database and playing program.