Hakon: brief but true. Let me elaborate:
Aragorn and King Arthur - the obvious parallel that comes to my mind is that both of them had a famous sword which represented their royal legacy and their right to kingdom.
(Ŕ propos, and elaborating on a side-thought in my last post: it might be interesting to consider which mythological motifs Tolkien did
not use; e.g. no sword retrieved by the hero from a lake, stone, anvil, tree or other receptacle reluctant to release the blade. Might tell us a lot about the difference between Tolkien and post-Tolkien commercial fantasy.)
Also, both Aragorn and Arthur came into their kingdom after a long time in rather infamous positions (although in different phases of their lives) - Aragorn as the Ranger Strider, Arthur as Sir Ector's ward.
Finally, both of them represent an archetype of the ideal King that appeals even to die-hard democrats like myself (but that would be another topic
).
Gandalf and Merlin - yep, the two archetypal wizards of world literature; I defy everybody to find a third! And both of them act as counselors to the Ideal King (see above). (Unfortunately, there's no Vivian/Nimue for Gandalf; but fortunately, this means he doesn't have to spend idle centuries banished into a haythorn hedge...
)
But what
Tuor is aiming at in this thread is parallels between Tolkien's Legendarium and classical=Greek/Roman mythology. The Norse and Celtic influences on Tolkien are quite well known and have been discussed any number of times (not that it can hurt to rehash them once more
, especially the Kalevala connection), unlike parallels between Middle Earth and ancient mediterranean mythology, which are still largely terra incognita.