Slug:
Two or more kids would pull sleeping bags over their heads down to their feet, get on their knees and then, with their hands behind their backs, attempt to bash and barge the other participants to the ground. That one could get painful.

Gods and Men:
Our own creation. One kid would play the part of a god of their choosing. The rest would be the men. The 'men' would lie down a few feet behind a sofa, and begin mocking whichever diety the 'god' kid chose to personify ("that Ra is such a cad", and so on). 'God' would then emerge angrily from behind the sofa and, balancing on the spine of the sofa, begin pelting the 'men' with scatter cushions. The aim was for the 'god' to remain balanced on the sofa for as long as possible as the 'men' returned fire with the thrown cushions.

Thornberries:
Don't even ask me how this one got started. The setup would be all of us sat in the far corner of a room acting like a bunch of obnoxiously well-behaved upper-class kids having a sleepover. Then, one participant would start to fake an uncontrollable bout of diarrhoea and be forced to leave the room, turning the light off on their way out so the rest of us did not have to look at the awful mess they had made on the floor. One or two minutes later, the diarrhoea kid would begin ominously scratching at the door, whispering satanically, and ominously grunting and sniffing. They would then enter the room and start shuffling/lurching/crawling their way toward the terrified group. When they got close, the following dialogue would take place:

"Do you know who I am?"

"No! Who are you?"

"I'M NIGEL THOOORNBEERRRYY"

commence screaming

Yeah so that 'game' didn't really have any win conditions. The aim was to just scare the other kids as much as possible and come up with ever creepier and more imaginative ways to play the role of Nigel Thornberry.