Written by Robert Frost in 1922, and published in 1923 in his New Hampshire volume.

According to Wikipedia, Frost wrote the poem in June 1922 at his house in Shaftsbury, Vermont. He had been up the entire night writing the long poem “New Hampshire”, and had finally finished when he realized morning had come. He went out to view the sunrise and suddenly got the idea for “Stopping by Woods on a Snowy Evening”. He wrote the new poem “about the snowy evening and the little horse as if I’d had a hallucination” in just “a few minutes without strain.”

In a letter to Louis Untermeyer, Frost called it “my best bid for remembrance”.

Whose woods these are I think I know.
His house is in the village, though;
He will not see me stopping here
To watch his woods fill up with snow.

My little horse must think it queer
To stop without a farmhouse near
Between the woods and frozen lake
The darkest evening of the year.

He gives his harness bells a shake
To ask if there is some mistake.
The only other sounds the sweep
Of easy wind and downy flake.

The woods are lovely, dark, and deep,
But I have promises to keep,
And miles to go before I sleep,
And miles to go before I sleep.

For those who believe that grammar and punctuation are truly important, this story is quite interesting:

An oft-repeated story holds that Frost wrote the first line of the last stanza without an Oxford comma: “The woods are lovely, dark and deep” and an editor or typesetter added a comma: “The woods are lovely, dark, and deep”. As can be seen (and as is pointed out by English literature teachers), the presence of the comma makes a significant change in the meaning of the line: “the woods are lovely because they are dark and deep” becomes “the woods are lovely, and dark, and deep.”

Frost is said to have ordered that it be removed. After his death, another editor (re)inserted it, in what critic Evan Lang Pandya calls a “gross besmirching of Frost’s intention that has gone down as one of twentieth-century editing’s foremost quibbles.”