ViZiGO is a client to play GO over Internet. It will let you play GO against thousands of players using any NNGS server (like IGS), and can be used to create and edit SGF files.
It runs under Linux and Windows, with source available under the GPL.
I like ViZiGO, but it has a habit of not letting you close a game window once it's finished (in windows, that is). --FCS