.app
I find it cool because it's one of the only ones that the abbreviation is actually a valid word and description of the file.
I hate .exe and .docx. x is not a good letter to have in a file extension. It just looks unfriendly. And how are you even supposed to pronounce .docx? "Docks"? "Doc X?" Uggh...
It's these little things that show the difference in attitude towards computing at Microsoft and Apple. Microsoft has uncomfortable 'x's all over the place.
Apple on the other hand has .app, .pages, .numbers - it just makes sense.
irmongoose