Ulmo

[Eurytus]

I do not have the books on me at present but I believe that in one of his letters Tolkien refers to the fact that there may have been at least one action by the Valar in the LOTR. Strangely in this letter he does not mention the seemingly more obvious things like the Ring being found by Bilbo or the Eagles rescuing Frodo and Sam from the destruction of Mount Doom. In fact, the incident that Tolkien mentions is one that he felt could have come from Ulmo and so is relevant to this thread.
When Frodo and Sam enter Mordor there is a moment when a stream of water, pure and unsullied, runs down to them allowing them to replenish their supplies. From the content of this letter it would seem that Tolkien felt this was a very subtle intervention from Ulmo.