Film Noir

I am going to start here with classic film noir, as opposed to remakes or new work (such as L.A. Confidential).

Strangers on a Train - this is one of my favorites: a Patricia Highsmith novel made into a movie by Hitchcock. It involves the story of two strangers meeting on a train and agreeing to murder the enemy of the other. It is copied widely because you will see a "strangers on a train" episode in many crime dramas. I saw one on a Criminal Intent recently, and they even referenced the movie.
Double Indemnity - this is considered by many to be the ultimate film noir, I think. Barbara Stanwyck is excellent at evil, and Fred MacMurray does a great job getting rid of his good-guy image.
Detour - Ann Savage gives the performance of her life here. It's a cheaply made movie by director Edgar Ulmer, but he gets a fantastic performance out of Ann. Ulmer made cheap movies away from the large studio system. Contrast the simple production (sets, clothes, etc.) with the performances.
Gaslight - is this really noir? It is to me. Ingrid Bergman is great, but Charles Boyer is why you watch the show.
Murder My Sweet - this movie is on my all time favorite list (across all categories). It is a great example of an early complicated plot, too. Dick Powell plays Marlowe here, which was unusual for him. He's great.
Touch of Evil - I don't know if this is noir either, but Orson Welles is a corrupt police captain and Charlton Heston is a Mexican. Get the version that incorporates Welles' notes (the original was not edited to his liking).