There was a discussion here in January about who the letter in the Way of Kings was addressed to (and whether it was actually addressed to Hoid and not written by him).
LINK
This was a pretty neat, and reasonably workable theory from what I can tell, not that I believe it is right.
However, I couldn't find any other topics about who it is addressed to.
And I think one thing that no-one has really mentioned was the "Ponder on that for a time, you old reptile, and tell me if your insistence on nonintervention holds firm." line.
Hoid is not a reptile, as far as I can tell (neither is Sazed, who has been suggested) - and it could be a figure of speech, but I am more inclined to think it is literal. Fantasy, intelligent reptiles that live to a long age... Lets not forget his planned series, Dragonsteel.
Would it make sense then, that the letter is written to a dragon?
Sorry if someone has already said this, I'm relatively new here (been haunting the Coppermind for a month or so).
On another (possibly unrelated) note, do we know/have we worked out what Hoid is up to? Brandon eluded in an interview somewhere that the evidence is there for us to find.
Assuming he wrote the letter - he is on a quest against Rayse (Odium), and/or Badavin (whichever shard he ends up holding) who are shattering shards (maybe not Badavin). Most of the times we see him he only plays relatively minor roles, although more than a few times it appears he is doing something larger in the background.
Actually, just found THIS thread which pretty much summed up what I was thinking.