Probably the best source in the world for go information is the Sensei's Library wiki.
Go Grinder a free program for working problems
Online Go Server Real Time and Turn based (HTML)
Dragon Go Server Turn based (HTML)
Internet Go Server (Java)
KGS Go Server (Java)
Samarkand this site has a great store and also has lessons by Janice Kim. Janice is a 3dan and the same person who does the lessons for the monthly shonen jump.
Yellow Mountain Imports this website has equipment for: Go, Mahjong, Chess, Chinese Chess, Shogi, Backgammon, Chinese Checkers, Domino & Pai Gow, Fortune Telling, Games, Cards & Casino Games, and other games. They also have a Tea Shop!
Yutopian eSTORE The Yutopian provides all kinds of equipment for your Go playing enjoyment. Everythings from Boards to Yunzi's are at your fingertips.
Yutopian eSTORE This website has many books that may be useful for you. they are for purchase and the prices are reasonable.
Slate and Shell Slate and Shell provides a plethora or reading material, although not as much as the Yutopian eSTORE. They are reasonably priced and are of exceptional quality.
- Go Problems this site specifically reviews problems that arise in Go. Namely Life and Death, Board Opening problems, and Endgame problems.
- The Go Teaching Ladder this site will review your games for you.
- rank tests find out your rank.
- The American Go Association Everything go.
- wikipedia has a great article on go.
- Computer Go a Wikibook on computer go.
- Go Base Go news.
- "Goama" Free Weekly Go magazine with Alexander Dinerchtein.
- Go4Go Professional game records, daily updates.