Will papers published in other sub-disciplines matter for graduate admission? I am currently a second year physics student from an Eastern European uni, that is thinking about doing a PhD in the US. I've done some research on the matter, but there are still a few aspects that I don't yet fully understand (graduate  system is rather different from Europe):
I saw that they value research experience of the applicants. I have co-authored a paper with a professor that has been accepted for publication in a national journal, one with another professor that will probably get published as well in a national journal and I may have found something that will maybe result in a paper in a more known, international journal. The thing is that this is not really "hot shot" research, i.e. not the kind of research that you would normally do for a PhD,  as I can't even tackle very advanced matters so I did something in areas that I can have a good grasp on (classical mechanics, finite time thermodynamics). 
My question is how will the admission board regard these papers? I mean it can't be bad, but will it give me an advantage over other applicants? 
Also, will it seem odd if I have research experience in a few unrelated areas? (as an undergrad I think I should explore as many subjects as possible)
Undergraduates are not generally expected to have already figured out exactly what they want to do in research, so it's unlikely that there will be any concern about the fact that you've worked in multiple areas.  The fact that you've been involved in research and published papers at all is a very good sign, even if it's not in exactly the field you are applying to be in: it means you're motivated and inclined toward research and have been able to accomplish significant things already.  In short: don't worry about it, but celebrate that you've got such a strong background!