I haven't seen too many yet. Mostly minor ones.
Ingame Guild Wars 2 every player, npc, and creatues has giant bobbleheads (example: http://i.imgur.com/zYcPbOU.jpg?1)
League has some new Arbitratore, I believe, where everyone becomes overpowered or something. http://euw.leagueoflegends.com/en/news/game-updates/features/evolution-league-legends-today
Cyborgmatt posted this video-sorcery-thing related to Dota, not really an april fool thouuugh.. For the nonbelievers. — Matthew Bailey (@Cyborgmatt)