According to lol wiki:
Law of Inverse Ninja Strength: Ninjas are more effective when they work alone. -1 health for every allied ninja. This applies only to the ninja characters: Akali, Kennen, and Shen.  This passive is cancelled when allied ninjas are all wearing their medical skins.
Prowl: Nidalee's Prowl grants her the power of a "cougar". Nearby allied champions gain +5 experience every 5 seconds if they are a lower level than Nidalee. 
Sunlight: Leona's Sunlight deals one less damage to champion skins wearing sunglasses (e.g., Vayne, Surfer Singed and the Commando skins).
Cassiopeia's hidden passive removes all snakes on the map. It is the one and only passive that alters the game field.
If Skarner stands still in a brush for twelve seconds, he may shout out a secret joke. This can't happen twice without leaving the bush he'd just stood still in.
When Graves's Smokescreen hits an opposing Nocturne, Graves will sometimes shout a special taunt.
When Fizz's Chum the Waters kills an enemy yordle (or small champion), the shark swallows the body, instead of the body lying on the ground till respawn.. This does not seem to affect Heimerdinger or Teemo.
Volibear and Zilean share a hidden passive, stemming from Zileas's opposition to an armored bear champion on the forums.
Volibear's hidden passive gives him a buff whenever there is a Zilean is on the enemy team that says "Chronokeeper Hater; Not even Zilean could keep an armored bear out of the League of Legends." He also shouts out special emotes whenever he kills Zilean. (also grants 11 extra gold from killing Zilean)
Zilean's hidden passive gives him a buff whenever there is a Volibear is on the enemy team that says "Armored Bear Hater; In my day, we would never have allowed an armored bear into the League of Legends." (also grants 10 extra gold from killing Volibear)
When the first Champion Spotlight was released for Lee Sin, it was stated that his hidden passive was blindness, which made the entire screen blurry while playing him at all times, but rendered him immune to abilities such as Teemo's Blinding Dart and Nocturne's Paranoia. However, this turned out to be an April Fool's Day joke.