xboard is a graphical user interface chess computer software
program for the X Window System. It is released as Free Software by Tim
Mann. It allows connecting to Internet Chess Servers (see: FICS, ICC),
using a computer chess engine such as Crafty, both of the above, or even
just viewing games. The Microsoft Windows version uses the same backend and is
called WinBoard.
You can download it at http://www.tim-mann.org/xboard.html if
you don't already have it (you might be surprised).