Source Forge is a resource for open source software development and distribution.
You can download lots of source material if you want to write your own go program
SourceForge.net is owned and operated by Geeknet, Inc., a publicly traded US-based company.
Website : http://sourceforge.net/
Source Forge is not the only place where you can find open source go software but it is likely the biggest.
Be aware that not all software mentioned here will work or even is still in development. (see the mentioned latest update) Also there is no garantee that this list is complete.
Recently (2010) Source forge has the option to give projects tags.
Tags to look for are: sgf, igo, baduk, go, baduk, gtp, go-text-protocol, weiqi