The original typical pairings were 1x2, 3x4, 5x13, and 13x6. However, you can pretty much throw numbers in a hat and pick out any two and be assured that someone has written something, some of it even worth reading. If you're interested in doujinshi, as opposed to fan fic, let me know and I can bring some down when I'm back this direction first weekend in December.

For het pairings, 1xRelena, 2xHilde (hasn't shown up yet), 5xSally, 13x11, 6xNoin, and occasionally out of a sense of completion 4xDorothy. For Trowa they dig into the back story and find a character from his origination side story, Midi Une (not to be confused with Treize's Une). I blame most of those on a spread in one of the art books, which showed each of the boys with their designated female counterpart. There is also occasional 3xCatherine, but many fans have issues with that one for spoilery reasons.