Scid vs. PC is a powerful chess toolkit. It has extensive database and PGN file handling capabilities, and can be used to play chess on the internet (with FICS), against a computer chess opponent, or for chess grandmasters to prepare for tournament matches. Additionally, Scid vs. PC can run automated computer tournaments with chess engines playing against each other.
Gameclock is a fairly simple application designed to track the time spent thinking by the players during a chess game. Various clock engines are available (speed chess, fisher chess, board games, or hourglass). The graphical interface is keyboard-driven and is intended to be minimal and simple. The code is made to be extensible to other game types.
GNU Chess lets most modern computers play a full game of chess. It has a plain terminal interface but supports visual interfaces such as X-Windows "xboard" and Windows-for-PC "winboard" as well as a full 3-dimensional wooden chess-board protocol for the Novag Chess board enabling one to be relatively free of the computer itself.