A collection of often-strange theories about the famous swords of Middle-earth.
Narsil: As discussed in the Deep Places, could Narsil be the sword of Haleth? One piece of supporting evidence is the other heirloom weapons of Numenor - the Axe of Tuor (Dramborleg), the Bow of Bregor, and the sword of Thingol (Aranruth). The first two are heirlooms of the Third and First Houses of the Edain - so a similar weapon from the Second House would very much be expected.
As for the issue with the length of Anduril - yes, Anduril is a long sword (probably some kind of hand-and-a-half blade, given that Aragorn uses it with a shield). But that doesn't mean Narsil was. It could easily have been reforged into something more suited to Aragorn's stature than the original Dwarven blade.
Glamdring & Orcrist (A): Who forged the most famous swords of Gondolin? We know that Glamdring was wielded by Turgon, and it seems likely that its mate was similarly made for the king, even if not borne by him. So who would forge the swords of the King of Gondolin?
One likely clue is that there are only five known glowing swords: Glamdring, Orcrist, and Sting (which form a set - perhaps Sting was a companion dagger to the swords), plus Anglachel and Anguirel, the black blades wrought by Eol, which burned with 'pale fire'. Eol himself wouldn't be found dead making swords for Turgon - but his son, Maeglin, carried Anguirel to Gondolin, and later became a master-smith who forged new and stronger forms of steel. He may well have known the secret of making them glow, and created a new pair of weapons as a combined demonstration of his art, and a gift to Turgon to earn favour.
Glamdring & Orcrist (B): And once they were forged... how did the sword of Turgon somehow wind up in a troll hoard in Arnor a few thousand years later? Assuming that an Orc pillaged them, it seems likely Morgoth would have taken possession of the blades - and we know he didn't head out east, because he stayed in Angband until it fell.
Except... one of Morgoth's greatest servants [i]did[/i] escape to the east: Sauron. If he took the swords with him, he could, 1600 years later, give them as a gift (as he was known to do) to a Numenorean lord, who would be suitably impressed by their heritage. And if that Numenorean accepted the swords, and the Ring that was given with them, then he would have fallen into Shadow, and under Sauron's influence - and, thousands of years later, as the Lord of the Nazgul, would have founded the realm of Angmar in the north. That puts the swords in the right area to be lost during the wars, and ultimately found and kept by three monstrous trolls.
Anguirel: Speaking of Anguirel... according to The Fall of Gondolin, Maeglin wasn't wielding a sword at the time of his death: one hand was holding Earendil (and got bitten by him), while the other had a dagger (and was broken by Tuor). Given that there was a war on at the time, we can probably assume he was at least wearing his sword, which would have fallen with him from the walls of the city.
Could it have survived? Glamdring and Orcrist were pillaged from the city itself, but Anguirel would have fallen into the fires with its master. To retrieve it before it was destroyed would have required a powerful servant of Morgoth, one with the ability to walk through fire without harm. There were creatures present at the fall of Gondolin who fit that description - the Balrogs, demons of flame and shadow. Could one of them have been rebellious enough to break off attacking to get a bit of looting in? Well... there is one who is notable for being independent enough to have abandoned its master at the end, and head off to nap under a mountain until woken by an infestation of dwarves. And it's interesting to observe that Gothmog, Lord of Balrogs, is only said to use a whip - while the Balrog of Moria, Durin's Bane, also bears a "red sword", that flames "like a stabbing tongue of fire". A glowing - nay, burning - sword, in the hands of the one being that could conceivably have retrieved it - is it too big a leap to see Anguirel here?
If so, its fate is appropriate indeed: for at the last, the sword of Maeglin, the traitor of Gondolin, is shattered by Glamdring, the sword of the city's lost king.
Gurthang: In contrast to its twin, the fate of Gurthang is well known: it broke beneath Turin when he died, and was buried with him beneath the Stone of the Hapless. Yet according to the Second Prophecy of Mandos, Turin will return at the End of Days, and wield Gurthang to kill Morgoth at last. In order to do that, the sword will need to be repaired - and before that, retrieved.
It is also a matter of prophecy, this time of Glirhuin, seer of the Haladin, that the Stone of the Hapless will never be defiled by Morgoth, nor sink beneath the waves. It is said that it still stands as Tol Morwen - and could it be that this is less in tribute to the House of Hurin, and more as a way of keeping the Black Sword available until it is needed again?
Another point: Tolkien was famously dismissive of the Arthurian legends, but still, his prophesied destroyer of evil is predicted to use a sword, not in, but under a stone...
Celeg Aithorn: What is Celeg Aithorn? Unlike the others, it's hardly a household name - but it might be the most significant of them all. It's found only in the Lay of the Children of Hurin, where Beleg names it in a spell of sharpness:
...the dirk of Nargil,
the knife of the North in Nogrod smithied;
the sweeping sickle of the slashing tempest,
the lambent lightning's leaping falchion
even Celeg Aithorn that shall cleave the world.
Celeg Aithorn gets the most description of all the blades in this passage, and its name suggests a high purpose: it's Gnomish (= early Sindarin) for 'Glass Sword of the Eagles', or more poetically, 'the Crystal Talon' (see The Fall of Gondolin for the invocation to "Arise O Thornhoth, whose beaks are of steel and whose talons swords!").
Who would wield a sword named for the eagles, described in terms of storms and lightning - and a sword which "shall cleave the world"? Surely the only option is Manwë, the Elder King himself. And, indeed, there are hints that his sword - though perhaps not Manwe in person - will be used for that exact purpose. The Book of Lost Tales tells that "Fionwë Urion, son of Manwë, [...] shall destroy the world to destroy his foe, and so shall all things then be rolled away", while a later passage describing Fionwë was amended to read "Eönwë to whom Manwë gave his sword".
(Thanks go to various members of the Barrow-Downs for helping piece this together.)