The Journal of Justice has this in spades. Along with giving extra info and backstory on the champions for the interested (quite a few) there are a number of "joke" articles which are just meant to be amusing. Special mention for Annie doing advertisements for dental hygiene. Yes, thatAnnie.
The Journal of Justice had an article, viewable on the wiki here, about Blitzcrank creating a sort of dating service. The article was amusing, but the best part was the accompanying illustration in which Blitzcrank shows photos of successful (well, he might have a different definition of it, being a robot and all) pairings from his service - one was Rammus and a cactus.
For Thanksgiving 2010, the Forums organized a AYAM game, or All Yordles All Mid day. At the start of the game all players must pay their respects for Urf the Manatee by dancing in front of his grave, then fight to the death in a 5 vs 5 match with no retreating allowed. Things get a little crazy.
The two blooper reel videos Riot made of their own Season One CG Cinematic. The first is worth a good giggle, with Groin Attacks and Epic Fail but the second is HILARIOUS, asking Fridge Logic questions like "what if the summoner controlling a champion is a klutz?", "aren't those minions Cho'Gath eats actually animated contructs?" and "just how dog-like is Nasus?"
