The Kiseido Go Server (formerly Igoweb) has some great Go resources, including this nice, short, interactive Go tutorial. (I love programmed learning.) You can also download the free 9x9 version of Igowin for Windows, from the maker of the full 19x19 The Many Faces Of Go. KGS, including the client and server software, is developed and maintained by the author of the original CGoban, that *nix Go board/client/editor program I mentioned the other day.