went looking for such a master list and turned up nothing. we'll have to make up this as we go, please correct and add to the list people.
Monster hunter tri: not required, but an option for finding players.
Mario Kart Wii: not required for matching people at random; friend codes help if you don't wanna play hackers!
Super smash bros brawl: not required for matching at random, but you don't know what you're getting.