Xgospel
X-Window client for internet go servers.

Complete Goban
Go program for Unix and X11. Local, internet or modem play, as well as .SGF-compatible record and replay.

TanGo
A freeware IGS and NNGS compatible Go client.

Ergo
Java-Applet to play against a Go-Server.

Java Go Client
A java plug-in to run Go against a server.

Jago
Java-Client, Internet Go Server client, a Smart Go Format go game viewer and a point to point go player.

CGoban 2
Includes an editor for SGF go files and a client for the Kiseido Go Server.

The gGo client
gGo is a Go board, SGF editor, client for the Internet Go Server and an interface for playing with GNU Go. gGo is written in Java and available for Linux, OS X, OS/2 and Windows.

GoKnot
GoKnot is an IGS telnet client and a Go player (playing locally against the GnuGo engine).