Fruits Basket.

I always recommend this one. It came out before Ouran High School Host Club.

Or Higurashi No Naku Koro Ni (if you don't mind gore)
Serial Experiments Lain.
Haibane Renmei
Nazca (if you can find this gem, it's old!)
Generator Gawl
Vision of Escaflowne

I have more I could list!