At Gen Con 2007 a fellow we met taught us this amusing six player Go Fish variant. I can't find any other rules online, so I'll document it here.
(2009-01-05 update: I've since found more information online, under the name Literature.)
Russian Fish requires six players exactly. Players are grouped into two teams of three players each. Since players want to share information with their own team, but not the other, players should be seated alternating from team to team. This encourages them to hide their cards.
The game uses a normal deck of cards with the jokers and 8s removed. The remaining cards for 8 sets, the 2-7 of hearts, 2-7 of diamonds, 2-7 of clubs, 2-7 of spades, 9-A of hearts, 9-A of diamonds, 9-A of clubs, and 9-A of spades.
The deck is shuffled and dealt evenly to all of the players, 8 cards per player.
Each turn the active player asks any player on the other team for a card by exact identity. If the other player has the card, he must give it to the requester. The active player stays the same and can immediately make another request to any player on the other team. If the other player doesn't have the card, they immediately become the active player.
When the active player, a player may attempt to take a trick. To do so, their team must have all of the cards in a set. The player must identify where the specific cards are. If the player is correct, they take the trick and put it aside. If they are wrong, the other team takes the trick.
When a team takes a fifth trick, they win. It is possible to have a tie game.
The game rewards paying attention and a good memory. However, you get do quite well only remembering cards in one or two sets you care about. To find the cards you're looking for, pay attention to who else is asking for them. Typically people won't ask for cards they have. If you haven't planned ahead with your team (which seems most fair for casual games), you can't be too clever in sending false clues, you'll confuse your own team just as much as the opponents.
The first team is Alan, Betty, and Chuck. The second team is Denise, Eva, and Frank. Alan starts, just because.
Alan asks Denise, "Do you have the seven of clubs?" She sighs and gives the card to him.
Feeling lucky, Alan asks her again, "How about the five of clubs?" "Nope," she replies.
Denise knows Alan has her seven of clubs. "Gimme my seven of clubs back." Alan does so.
Denise notices Alan was looking for low clubs, so she guesses he has at least two of them. "Do you have the six of clubs?" she asks him. "Yes," he says as he hands it over.
Not wanting to press her luck, Denise looks elsewhere. "Betty, got a king of diamonds?" "Go fish."
Betty turns to Eva and asks, "Can I have your nine of spades?" "If only I had one."
Eva takes a long shot and asks Alan, "Do you have a two of clubs?" Alan scowls and hands it over. Eva scans he hand. She has the two of clubs from Alan and was dealt the three, four, and five. She announces, "I know where the low clubs are. I have the two through five. Denise has the six and seven." Sure enough, that's correct, and her team has a trick. The clubs are set aside as a set.
(Thanks to the fellow whose name I forgot for teaching us. Thanks to Curtis Rueden for reminding me what happens if you mis-guess where the cards in a set are.)