Curses!

[Maédhros]

There is an interesting curse uttered by Túrin, when he mets Mablung in the final stage of the Narn in which he curses Menegroth.
From the Narn

"They went out into the wild seeking you," said Mablung. "It was against all counsel; but they would go to Nargothrond, when it was known that you were the Black Sword; and Glaurung came forth, and all their guard were scattered. Morwen none have seen since that day; but Nienor had a spell of dumbness upon her, and fled north into the woods like a wild deer, and was lost." Then to the wonder of the Elves Túrin laughed loud, and shrill. "Is not that a jest?" he cried. "O the fair Nienor! So she ran from Doriath to the Dragon, and from the Dragon unto me. What a sweet grace of fortune! Brown as a berry she was, dark was her hair; small and slim as an Elf-child, none could mistake her!"
Then Mablung was amazed, and he said: "But some mistake is here. Not such was your sister. She was tall, and her eyes were blue, her hair fine gold, the very likeness in woman's form of Húrin her father. You cannot have seen her!"
"Can I not, can I not, Mablung?" cried Túrin. "But why no! For see, I am blind! Did you not know? Blind, blind, groping since childhood in a dark mist of Morgoth! Therefore leave me! Go, go! Go back to Doriath, and may winter shrivel it! A curse upon Menegroth! And a curse on your errand. This only was wanting. Now comes the night!"
Then he fled from them, like the wind, and they were filled with wonder and fear. But Mablung said: "Some strange and dreadful thing has chanced that we know not. Let us follow him and aid him if we may: for now he is fey and witless."

It makes you wonder, did Túrin's curse had any real effect on the Ruin of Doriath? Or was it just his bitterness towards them for the fate of his sister and mother.

[Nilpaurion Felagund]

It makes you wonder, did Túrin's curse had any real effect on the Ruin of Doriath? Or was it just his bitterness towards them for the fate of his sister and mother.(Maédhros)

Perhaps both. After all, Túrin was devastated at being the cause of his sister's death, and has this terrible habit of lashing out against those who bring him closer to his Doom - Brandir was a good example.

And the Nauglamir, one of the causes of the ruin of Doriath, was delivered by Húrin from Nargothrond, as "payment" for the "fair keeping" of his son. Túrin killed himself due to Mablung's words. Húrin saw that, and probably was angry at Mablung - and Doriath, for many other reasons aside from this. So he adds this little item to his "Things to do once I escape this place" list:

~ Bring something to reward the king of Doriath for "taking care" of my family. (If it wasn't for him - and his subjecs - all of them would still be alive.)

But another thing I ask here...who effected the curse on Húrin's hîn? children, I mean. All evidences point to Morgoth - through Glaurung - but...

[Húrin: ]...you cannot see them, nor govern them from afar, while you [Morgoth] keep this shape, and desire to be a King visible upon Earth.

(UT I 2)

[Tuor of Gondolin]

And what of Eol's curse on his son (twice):
" 'Come, Maeglin son of Eol! Your father commands you. Leave the house of his enemies and the slayers of his kin, or be accursed!' But Maeglin answered nothing.".....

"So you forsake your father and his kin, ill-gotten son! Here shall you fail of all your hopes, and here may you yet die the sdame death as I!"

And how are free will and curses intertwined?

[The Saucepan Man]

And how are free will and curses intertwined?

Ah now, that one's a biggie.

As I recall, there is quite a good discussion of this issue in the context of the curse laid on Hurin and his family here:

Hurin and despair

Edit: I hasten to add that I am not suggesting that discussion of curses in the context of free will should not continue here. I am merely providing the link as it is relevant to the issue raised.