I try to bring something for a couple of reasons. One, becuase if I don't then I feel like a mooch. And second is that I'm choosy on what I drink, so if I bring something (ie Cider) to a party I know I'll have at least one choice.

But if I don't have the chance, or money, to pick something up I know that my friends would rather have me come empty-handed than not come at all.

When I'm hosting I don't expect nor require anyone to bring anything. That way it's all a pleasent surprise when they do. :)