I have made an online 2 player game of Go here:

http://dev.farver.net/go/

It is a two player game (human vs human) where one player sets up a game (grid size, komi, handicap etc.) and generates a challenge key to send to a friend.

The game is asynchronous so that you can sign out and continue the game at a later date.
You also have a list of all your previously played games, you can publish your games and view other public games in the archive etc.

Playing the game seems to work fine, though I would be pleased to hear any comments you may have. However, the points-counting in the end is the difficult bit as far as programming goes. Not least because I am not so sure how to count points my self, let alone have a computer do it.

I have implemented points like this:
Chinese points: pieces on the board + area controlled (+ komi for white)
Japanese points: prisoners taken + area controlled (+ komi for white)

Anyway, I hope you like it and please flag any bugs, problems or mistakes here.

Cheers