Yahoo! (The search engine/portal) runs a little Go Server of their own. Many people compare it to Hell (hence "Yahell"). Quite a few people have taken to cheating on their servers as well.
I'm not sure of the algorithm their rating system uses, but it is largely incorrect due to the number of cheaters and sandbaggers present. Also, it is common practice to escape a lost game.
All in all, "Yahoo! Go" gets a collective thumbs down from the serious Go community.
Barthoze: There are lots of things wrong with "Yahoo! Go" Server.
I've moved to IGS now, and I won't go back there.
The "stupid passing rules" are as follows: after two passes, players select which stones are dead, and then a prompt comes up to ask whether the players agree that the game is over. (Assumably to prevent people from marking undead stones as dead to cheat.) If the the players do not agree that the game is over, the game continues, and passing is forbidden for the first player at this point.
This means that in many situations a player can force his opponent to, effectively, never pass. (By cancelling out the pass by passing and then not ending the game.)
Funkybside: Yes but I'll give yahoo credit for one thing; Had it not been for how horribly bad yahoo go is, I would have never found IGS. For that, I am greatful!
Yahoo does have a wide appeal. It certainly gets people interested. I used to be an avid backgammon player. One night, tired of backgammon, i tried out go. I haven't gone back.
Artis: Not just "not ending the game", but possibly letting your (frustrated) oponent to run out of time. Someone pulled this on me once. I asked him why he did it -- he said he wanted to win...
Malweth: Yahoo does have its good points. Most games are even and you never know your opponent's strength (true, this is a side effect of the bad ELO system). This gives you more versatility in playing players with different strengths. Furthermore, at the approx 8k level you have about an equal chance of playing a weaker player as you do someone stronger. Playing someone stronger while thinking that they may be equal to you is a good way to get stronger yourself. It's also interesting to note that you can play go on Yahoo Japan - but unless you read Japanese it isn't easy trying to figure out how to make an account.