XBoard is a graphical chessboard for X that can serve as a user interface to play against any of a variety of chess engines that run on your local machine, or against human or computer opponents on an Internet chess server. XBoard can also be used by itself to read and write game files in PGN (portable game notation). WinBoard is a port of XBoard to Microsoft Win32.
XiStrat (aka 'Extended Strategy') is in particular about turn-based, networked multiplayer, non-cooperative, zero-sum, abstract strategy board games (e.g., Chess, Go, Reversi variants, etc.) on 3D-visualized polyhedra and contains a server, client GUI, autoplayer engine, utilities, and documentation. Related recreational modern mathematics (single agent, cellular automata, graph/group/complexity/knot theory, discrete geometry, algebra, combinatorics, and mathematical physics) is also dealt with.