This is mostly in reference to this post about the reason Boromir travelled to Rivendell in time for the Council of Elrond. Both he and Faramir had a prophetic dream that they wanted Elrond to interpret for them.
There are a few theories as to how they got the dream. The easy answer is that it was Fate. Then there are some who say it was Iluvatar. Personally I think that’s unlikely, since Iluvatar isn’t really known to get involved personally in little stuff like this.
I think the most likely option is that it was the Valar. They did send the Istari for the sole purpose of countering Sauron, I think it’s believable that they’d send “suggestions” to get important people where they needed to be for the same purpose.
Then there are some who say it was Gandalf. This is also possible, especially since we’re told in The Silmarillion that one of the Valar Gandalf studied under was Lorien, who is in charge of dreams. Some people also think it could have been Elrond, but I don’t think it’s likely. Faramir and Boromir had their dreams months before the council, and Elrond wasn’t yet aware that the ring had been found. Also, I’ve never seen anything to suggest that Elrond has the power to give people dreams.
SOURCES: LOTR, The Silmarillion