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.