Another classic worth mentioning is "The Hidden Fortress." I've only watched half so far, but supposedly this Kurosawa Japanese film is where George Lucas drew much inspiration for Star Wars, and C3PO and R2D2.
Not supposedly, it DID have a major influence on Lucas. I have the Criterion dvd for Hidden Fortress and there is an interview with Lucas where he talks about how the film influenced him when he wrote A New Hope.
The two bickering peasants in Hidden Fortress are clearly the inspiration for C3PO and R2D2 and the princess they encounter during their journey is clearly the inspiration for Leia.