Software for connecting to go servers.
An sgf editor and IGS client for the Mac.
An IGS client and game viewer.
An SGF editor and IGS/NNGS client, originally written by Bill Shubert, and now continued by a group of people as a Sourceforge project.
An IGS client and SGF viewer/editor, written by Rene Grothmann.
qGo is a Go client and full featured SGF editor, available for Linux/Windows and MacOS X. qGo code is C++/Qt and it's free.