Go Games, Go Information and Go Study Tools

A couple Go resources that even people who follow Go might not have stumbled upon:

http://www.gobooks.info/bib-difficulty.html

http://www.audiogolessons.com/

http://www.aygoschool.com/Ang-Yue_guide.pdf

Also, while I don't advocate violating copyright when it's possible to legally purchase the material, Scribd has PDFs of most of the famous English-language books if you want to check them out. There is also a .torrent that is basically everything on Scribd in a single download. Of course you can also get all the books here:

http://kiseido.com/go_books.htm

I'll toot my own horn and share my site, which started as a study tool for myself but that many people have also found useful (it's open source):

http://eidogo.com/

You can study joseki, search for board patterns in pro games, play GNU Go, etc. The board part is modular and can be embedded into any webpage. Like GoBase, it has a pro game archive, but does not require registration or payment to view.

Also:

http://www.goproblems.com -- a great site with thousands of problems to solve

http://senseis.xmp.net -- a Go wiki with a lot of interesting articles especially for the intermediate player

I've seen mention of KGS (kiseido go server) but not IGS (internet go server). I play on IGS because generally the players are a little bit stronger there (not that I am ;-) ).

http://www.pandanet.co.jp/English/