A forest of about 400 pine trees in Western Poland all grow with a 90 degree northward bend at the base of their trunks. The patch, within a a larger forest of straight growing pine trees, was planted in approximately 1930, and it is assumed that their peculiar growth habit is due to some mechanical intervention, though the reason behind it is unknown. A commenter on the original post (at discoverynews) said he was taught to do this by his grandfather, with the intent of making saplings grow ready-shaped for canes. So perhaps this was a cane forest interrupted by World War II.
The twisted trees of Saskatchewan Canada are more mysterious. The grove of deformed aspens is on private land, and though the Friends of the Crooked Bush speculate that the trees could be due to meteorites or even UFO's, a more likely explanation seems a rare genetic mutation such as that causing contortion in the Henry Lauder's Walking Stick (Corylus avellana 'Contorta'). When vegetatively propagated and grown at locations in Manitoba, the Saskatchewan aspens retain their crookedness.
But my favorite crooked tree story is this one from my home state of Oklahoma, and the Land Run town of Shawnee (which oddly enough also happens to be the birthplace of Brad Pitt):
"In a whimsical moment" Shawnee residents Frank Witherspoon and Gule Rinneger went down to the banks of the North Canadian river, dug up two elm saplings, and brought them back to town in a one-horse hack. Witherspoon decided that he would form an arch of the two trees by tying them together in front of his newly built house. Witnesses said that the plants were more than six feet tall, and that he tied them together as high as he could reach, using ropes and burlap to bind them. In spite of the mischief of neighborhood children, who used to cut the bindings, he was successful in his efforts to grow the trees into a knot.[source]
They grew more closely attached through the years, bending together with age. In 1930, their picture appeared in the syndicated "Believe It or Not" column of Robert Ripley, and again in the book "Nature Woodland Wonders" in 1945. The Oklahoma state highway commission included them in its booklet, "New Thrills Ahead." at about the same time; they were by that time just a few feet away from State Highway 270 and a regular stop for travelers. I can't find any information on when they went at last; but I'm sure they went together.
There was a crooked man, and he walked a crooked mile.
He found a crooked sixpence against a crooked stile.
He bought a crooked cat, which caught a crooked mouse,
And they all lived together in a little crooked house.