Uh-huh.
I think this is an American trait, or perhaps I simply never noticed it in Australia (perhaps my Australian readers can give me the definitive word on this), but when I say "Thank you" or "Excuse me", the usual response is "uh-huh", delivered in a disinterested fashion that I interpret as "whatever". Personally, I'd prefer silence over "uh-huh" if it's too much to expect a simple (and polite) "You're welcome" or "No problem".
Of course, the other reason I bring it up is that now I'm doing it too and that grates on me :).
Comments
Nope, I don't ever recall hearing people say "uh-huh" instead of thanks in Australia. Actually I don't think I've even heard it anywhere outside Seattle.