action:

Fight Club, Heat

adventure:

Raiders of the Lost Ark, North by Northwest

animation

Shrek, Monsters Inc

comedy:

Groundhog Day, Le Fabuleux Destin d'Amelie Poulain, Liar Liar, Oceans Eleven, SAVING RYAN'S PRIVATES;)

crime:

The Usual Suspects, Pulp Fiction, Reservoir Dogs, Casino, Donnie Brasco

documentary:

Bowling for Columbine, The Celluloid Closet

drama

Once upon a time in America, One Flew Over The Cuckoo's Nest, Forrest Gump

fantasy:

The Two Towers, The Fellowship of the Ring, Donnie Darko

film-noir/neo noir:

Chinatown, L.A. Confidential

horror/scary movies

Psycho, The Shining

musical:

Wizard of Oz, Sound of Music

mystery:

Memento, The Sixth Sense, Se7en, The Conversation, Following

romance:

Casablanca, Rebecca, Titanic

science fiction:

The Truman Show, A Clockwork Orange (and Star Wars if you're into it)

suspense/thriller:

Rear Window, Silence of the Lambs

war(related):

The Pianist, La Vita E Bella, Schindler's List, Saving Private Ryan, The Deer Hunter, Full Metal Jacket

western:

Sergio Leone's pictures

historical:

Spartacus!!, Gladiator, Braveheart, Gangs of New York