Thief is described as 'free chess client written by Zon Jan to play on chess servers like the Free Internet Chess Server (FICS). Thief can be customized and supports regular chess and all variants like bughouse, crazyhouse, Losers, Suicide, Kriegspiel and others' and is a Chess App in the Games category. There are more than 25 games similar to Thief for a variety of platforms, including Windows, Android, Online / Web-based, Linux and iPhone. The best alternative is Lichess, which is both free and Open Source. Other Thief like games are Chess.com (Freemium), XBoard (Free, Open Source), Arena (Free) and Shotgun King (Free).
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XBoard, also known as WinBoard on Microsoft operating systems, is a free graphical user interface client developed by Tim Mann that is compatible with various chess engines that support the Xboard Communication Protocol such as GNU Chess.
Arena is a free Graphical User Interface (GUI) for chess. Arena helps you in analyzing and playing games as well as in testing chess engines. It runs on Linux or Windows. Arena is compatible to UCI and Winboard protocols.
Jin is an open source, cross platform, graphical client for chess servers, written in Java. Jin can run either as a standalone application (available at this website) or as an applet, directly from your browser (available at the chess server's website).