An interface for FICS. Quite featureful. GPL, Unix/Linux only.

A Java Applet allowing to play on the Free Internet Chess Server. Not very featureful, but gets the job done if you can't install an interface. Freeware, Platform independent.

Knights is a relatively new graphical chess interface for KDE. It is, however, already quite featureful and very nice looking. Doubles as a chess engine front end. GPL, Unix (KDE) only.