Yay- always good to see another crossover junkie, here are just the ones I have on my faves:

HP//OHSHC: http://www.fanfiction.net/s/4389264/1/ (MoriHarry pairing, gradual development, very sweet)

HP//OHSHC: http://www.fanfiction.net/s/3471521/1/ (Gen as far as I can remember, lots of humour, HC goes to Hogwarts and chaos insues)

TRC//Lots of crossovers: http://www.fanfiction.net/s/3009229/1/ (Only one OHSHC entry but all of the mini stories are worth a look at)

OHSHC//Clamp crossover: http://www.fanfiction.net/s/3186186/1/ (Brilliantly written and look for other similar stories in their profile, though they have more Clamp that Ouran)

OHSHC//Death Note: http://www.fanfiction.net/s/4425581/1/ (Short but funny, Evandar has lots of good stories, including a Creature!Harry one)

OHSHC//Kaito Kid or Dectective Conan//others: http://www.fanfiction.net/s/3912154/1/ (Five schools Kaito Kid did not attend- made me lol)

OHSHC//DN: http://www.fanfiction.net/s/4057768/1/ (Ouran's take of the Death Note.)

OHSHC//Fruits Basket: http://www.fanfiction.net/s/3395323/1/ (Yuki is the new 'ice prince' host)

OHSHC//DN: http://www.fanfiction.net/s/3313650/1/ (L, meet Host Club, Host Club, meet L)

OHSHC//FB: http://www.fanfiction.net/s/3162933/1/ (Lots of twins, Touru and Pumpkin Pie)

That's all I can find at the moment, I hope you haven't read them all- enjoy! ^_^