Alfred Hitchcock was a famous iconic British filmmaker of the 20th century. He made many famous thriller/horror movies like 'Psycho' and 'The Birds'. He has also had a cameo in some of the Treehouse of Horror episodes.
Alfred Hitchcock first appeared on the Simpsons in Treehouse of Horror XX in Dial M for Murder. He is seen getting off a bus as Bart flees from Lisa. This is similar to his appearance in 'North by Northwest' when he just missed the bus he was trying to catch.
Alfred Hitchcock is also seen in the famous Treehouse of Horror XXIV opening. He is outside Springfield Elementary School feeding some crows (or ravens). He then throws some birdseed onto Edna Krabappel. The birds then attack her. This references Hitchcock's movie, 'The Birds'.