Can Superman lift or otherwise move Thor's hammer?
Well Superman appears to be from a post singularity civilisation given that his powers are beyond the understanding of any known science, then again if his race was comprised of beings like him it begs the question, how we're they destroyed? What seems more likely (I'm not up on his present origin story canon) is that he was genetically modified or whatever as a desperate attempt to save, or at least leave some record of his race. In which case if we compare Superman to Odin there's clearly a disparity, as prodigiously powerful as the former may be the latter is the one who likely has access to superior technology, thus we come to the matter of how Thor's hammer is secured to the ground. If it's merely extremely heavy (I didn't see the movie) due to some localised Higgs Boson field or something then it's probable that Superman could lift or at least drag the hammer about, if not practically use it, whereas if it's physically locked in space by some kind of quantum anchor (I'm really making this shit up as I go) then there's every reason to believe Odin is using the technological/logistical power of his empire in some way, in which case no matter how strong Superman may be Odin can just add more power to the inertial dampening field so that even if the Sun went supernova that hammer would still spiral around in Earth's orbit, completely unaffected.
Could he do so if Thor wished him to? Could he do so if Thor did not wish it?
Depends if Odin has removed the child lock.
Could a lightsaber cut through Thor's hammer?
Not if it's designed to be heat resistant (which I assume it is, being a weapon and all) I mean we could make materials that are at least partially impervious to a lightsaber and with Odin's technology Thor's hammer could have some kind of sub-space heat sink so no matter how long you hold a lightsaber to it the hammer itself will never become too hot to hold.
Could Yoda move the hammer by using the force (or could any other jedi or sith)?
Yoda at his best was no Superman (though I'd still pay to see the two fight) and though there are various other significantly more powerful force users it's still doubtful any one of them could move something locked in place by technology clearly far beyond their understanding.
Could Harry Potter (or any other wizard) move it with the accio charm?
It depends how magic works, I don't think Harry could but Voldemort or Dumbledore might be able to bend time/space around it so much that Odin would lose his lock on it, I mean the screwed up thing about magic is that it has no clearly defined limits, even after hundreds (thousands?) of years the wizards of the Harry Potter universe just seem to be screwing around with forces they don't really understand, and from time to time magic seems to make up or change its own rules for inscrutable reasons, e.g. Harry releasing the snake in the very beginning by making the glass disappear with absolutely no idea how he did it, an ability that never appears again.
If Buffy needed to get the hammer to Thor, could she lift it and get it to him? If she wanted to lift it for some other reason, could she?
Though I believe slayers are empowered by magic, that appears to be the extent of it, and as strong/fast as she may be I don't think she ever reaches Superman levels, then again her witch friend would comfortably fit in the Harry Potter universe so together they might be able to figure something out.
Could Dr. Manhattan move the hammer?
He breaks every rule twice over.
Could Q (from Star Trek, not James Bond) move the hammer?
Ditto, rule breaker, a better question would be if the crew of the enterprise could figure out a way, and I suspect that if given enough time (probably decades) they would eventually reverse engineer Odin's technology, if Odin himself doesn't intervene first.
Could Thor's hammer be used to destroy the One Ring?
Hard to say, it's magic after all and it's strongly implied that the One Ring can only be destroyed in Mt Doom where it was forged, not just in any other volcano anywhere else on middle earth or the absurdly hot fires that could be achieved by a combination of elven black smithery and Gandalf's magic, also when stuck during the council meeting the dwarf that struck it was repelled as if the striking force was reflected back, so I think hitting it with Thor's hammer (which strikes hard enough to daze the Hulk) would be a very bad idea.
Could Thor's hammer kill Freddy Krueger?
Given his magical and incorporeal nature I doubt it, however if attacked by Freddy Krueger I suspect Thor's heroic willpower will enable him to manifest an in-dream equivalent and that would be enough to at least banish Freddy, but not kill him as I believe he's an already dead malevolent spirit or something like that.