I'll throw in one that I use.
Players can perform actions with up to 3 other players, (4 at the max) such as checks, if the players have +-3 of the skill. (Meaning that someone with 10 streetwise can only help someone with 12 or 8, ect). The more people that contribute, the more likely that it will either succeed, or cause mayhem. With two people, I make a separate roll on a D10, if the roll is 1-2, then it fails, and it basically turns into a critical fail. but, if it is an 9-10, then it succeeds. With 3, it is 1-4; 7-10. And 4 is 1-5, 5-10. So it turns into a question of chance. It could be a bit more balanced, but when it fails, it fails hard.