interactive Go tutorials (using java applets) in many languages
the perfect place to learn the game =)

a Wiki about Go, probably the best Go-page on the net:

gGo, an excellent free Go client:

the Kiseido and the Internet Go Server
meet hundreds of other players online, including professionals ^_^
(if you aren't familiar with the ranking system, use 28k as initial rank)

a huge database of Go problems
you have to register and login to view the page in english