Lichess - The complete chess experience, play and compete in tournaments with friends others around the world.
ChessDB - ChessDB - a free Chess database for Mac OS X, Windows, Linux, and UNIX - like ChessBase, but better
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.
Chess Tempo Database - Chess Tempo Database gives you a library of more than 2 million searchable chess games.