Links
The "Big Three"
goproblems.com
Goproblems.com is probably the Nr. 1 source for free online tsumego. You can solve tsumego against the clock according to your level. There are numerous additional functions such as viewing how most people would start solving a problem, which is often very interesting.
The Go Teaching Ladder
Several high dan players have recommended getting your own games reviewed by stronger players as a good way to make progress. However it is not always easy to find stronger players willing to comment your games. The Go Teaching Ladder has been set up for this purpose. Volunteers in the ladder comment games made by weaker players. Anyone may submit a game for commenting by a stronger player.
Sensei's Library
Sensei's Library (short: SL) is meant to be a place where Go players can meet to find information, contribute information and discuss any items related to Go.
The concept is similar to Wikipedia. Sensei's Library is mainly in the English language, but articles in other languages are also welcome.
Submit a Link
If you are the owner of a go-related website or just discovered a great new one feel free to write me an email. I will then consider placing your link here. Backlinks are, of course, greatly appreciated.