One of the features of the Zombie Brand legendary skin grants him a unique run animation and movement dialogue while chasing an enemy champion.
When Draven catches an enemy Draven's Spinning Axe, he will shout a special emote instead of his usual line whenever he catches his own Spinning Axe.
When Chum the Waters kills an enemy yordle, if they're small enough, or any other small champion, the shark swallows the body, instead of the body lying on the ground till respawn.. This does not seem to affect Heimerdinger, and Teemo.
When Graves's Smokescreen hits an opposing Nocturne, Graves will sometimes shout a special taunt.
While using the Jaximus skin, Jax will occasionally say: "Here's to you, kid." This is a memorial tribute to Joe, a Ewing's Sarcoma patient who visited Riot thanks to the Make-A-Wish Foundation.
Sunlight deals one less damage (before calculating MR) to champions wearing sunglasses. This includes all Commando skins, Surfer Singed, Classic and Aristocrat Vayne, Vandal Twitch, Vandal Gragas, Safari and Officer Caitlyn, Dragon Fist Lee Sin, Corporate Mundo, Hired Gun Graves and Riot Graves. Vi and Neon Strike Vi take their sunglasses on or off when they use their taunt, and receive this easter egg so long as they are on.
This has been officially stated in-game as a Did you know? trivia fact.
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. 
While Thrill of the Hunt is active, Rengar will sometimes shout a special taunt if he reveals an opposing Kha'Zix using Void Assault.
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.
Force of Will gains a bonus effect when used on the Ancient Golem or the Lizard Elder.
*Upon throwing the Ancient Golem, Syndra will replenish 10 mana and the cooldown of Force of Will is decreased by 1 second.
*Enemies hit by the Lizard Elder are afflicted by the Blessing of the Lizard Elder.
When Twisted Fate's Gold Card stuns an an opposing Graves, Twisted Fate will sometimes shout a special taunt.
★Volibear and Zilean★
Volibear and Zilean share a rivalry, stemming from Zileas' opposition to an armored bear champion on the forums.
Volibear's hidden passive grants 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 and receives 11 extra gold.
Zilean's hidden passive grants 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." and receives 10 extra gold from killing Volibear.
Zyra has 3 hidden passives.
*When hit by Leona's Sunlight passive, Zyra's character model grows slightly, as do her plants.
*When Maokai is very close to Zyra, he gains 1 movement speed.
*When Zyra or Sion kill one another, they will get 2 additional gold.
League Of Legends Secret Passive List