GoBase
GoBase is a website owned by Jan van der Steen that contains a large collection of professional games, as well as a number of articles, including study material.
Between roughly 2009 and 2014 the site was no longer actively maintained. However it was updated quite substantially in January 2015.
It has a database search engine for searching through the collection and includes an online replayer that can display the games found in the database.
A large part of GoBase, including the replayer, requires an username and password to access. Registration for the username and password was free. As of 5 August 2007, there is a 20€ fee in order to register an account. Registration must be done from a non-anonymous and non-free email account, failing which extra credentials must be supplied.
See also 321 Go which is described by GoBase as its 'little brother'.