ChessDB - ChessDB - a free Chess database for Mac OS X, Windows, Linux, and UNIX - like ChessBase, but better
Lichess - The complete chess experience, play and compete in tournaments with friends others around the world.
XBoard - XBoard, also known as WinBoard on Microsoft operating systems, is a free graphical user interface...
DecodeChess - AI chess tutor and analysis
Chess.com - Play chess on Chess.
ChessPad - ChessPad is a feature-rich chess game viewer and PGN file editor available for Windows.