Links:
Go Wiki: Contains lots of useful information about Go including its history, its different rule sets, equipment, Computer Go, terms, and tournaments.
American Go Association: Head organization of Go in America. Determines player rankings in America, has listings of most Go Clubs and tournaments, lists news in the Go world, and has a useful resources page for links and equipment.
Sensei's Library: A collaborative web site about and around the game of Go (also called Igo, Weiqi, or Baduk). It is a repository and a discussion medium for Go.
Go Base: One of the most detailed and professional looking Go web pages, contains news and databases for many pro games, tourneys, and players. However, registration requires a mandatory "donation."
Go4Go: Free online database for professional Go games. The layout is not as organized or asthetic as Go Base's, but Go4Go's registration is free.
goproblems.com: Free Java based Go problems for all skill levels that allow you to input answers to questions and see animated solutions.