Rui Mendes Homepage

The interactive way to Go: An interactive tutorial of Go, with some exercises to explain some of the concepts.
Sensei's Library: A very good site with information about Go. A bit overwhelming for complete beginners.
How to play Go: Another introduction to Go.
GoBase.org - Go Games, Go Information and Go Study Tools: A good site about Go. Look at their collection of Go problems as well.
The Go teaching ladder: This site has a very interesting service. You can upload an sgf file of your game with questions and the reviewers will review and comment your game for you.
Kiseido Go Server
Go Bibliography
Go Problems: A very nice collection of problems.
Associação Portuguesa de Go

Go