No sir

Aragorn met Arwen?( Once the Ring was destroyed, Elrond and his family would have left Valinor not long after.)

openay! orrysay! :D BTW, that's nope, sorry in Pig Lattin!

The Free Peoples united in The Fellowship to destroy the Ring?

Sorry, but your not correct

Bilbo would have died in the tunnels under the Misty Mountains?

Correct! Congraduations!

Wow, really?

That caught me off guard, so I will start with a fairly easy one.

Maedhros was taken captive by Morgoth

The exiled elves were reconciled?

Correct, Tuor of Gondolin

How about:

BB forgetting to send Gandalf's letter to Frodo?

Because they left the Shire later than Gandalf intended, the Hobbits met Gildor and his companions, who were able to send word to Elrond in Rivendell of Frodo's plight, thus ensuring that Glorfindel was abroad searching for them. But for this chain of events, Frodo would almost certainly have succumbed to the Witch-king's blade.

An excellent, well-thought out answer. But not what I'm thinking of. :)
And also, if Frodo had left earlier the Nine wouldn't have been in Eriador when he
was going to Rivendell. (Which would have led to a rather boring trip to
Rivendell). :D

Aragorn came with them?

they met bill the pony? :D

Aragorn. Nope. I believe (?) he was going to meet the hobbits and
Gandalf anyway.

Bill the Pony? Nope. Actually, that is a possible answer, but not the specific
positive thing (at least for many people) I'm thinking of that was an eventual
plus thing that happened later relating to the letter not being sent on time.

Frodo's delay due to the letter not reaching him on time gave the 'conspirators' a chance to formulate a plan to go with him when he left. If he had left earlier, Merry, Pippin and Sam would not have gone with him. All three played important roles later, but the main one that comes to my mind is that Pippin was involved in the death of the Lord of the Nazgul.

:) Yoo-hoo, Tuor of Gondolin. Is anyone close to the answer, or do we need a nudge in the right direction?

Sorry about that.
The answers above are actually much better then the rather
frivolous one I have in mind.

A clue:
It certainly helped BB professionally.

Gandalf blessed Butterbur's beer?

Correct.
Although it seems a questionable boon, since Barliman was, in effect,
rewarded for screwing up having Gandalf's letter forwarded. :)

How about Morgoth cursing Húrin & co?

Glaurung was slain? If they hadn't been killed, Turin would probably never had been sent to Thingol, and would probably not have ended up killing Glaurung.

Correct :D

Anyone can have the thread...

ok, I'll take it
 

The curse: Sam almost drowned.

Interesting thread
 

The blessing: Frodo had a companion on his journey?

yup.

The curse: Nimrodel is lost on her way to the havens.

Amroth was lost at sea, letting Celeborn and Galadriel become Lord and Lady of Lorien, so they could guide the fellowship and help frodo