Well, you could probably write a book on that subject, I'll throw out the following: Repulsion (cited by Romero), Black Sabbath, Blow Up (basically ripped off by de Palma in Blow Out), The Wages of Fear (influenced Friedkin), 8 1/2 (a favourite of Scorsese, though I don't necessarily see it in his films), Peeping Tom, Les Diaboliques, Rififi, Contempt, Persona, Z, Le Samourai.