Okay, so its not exactly a house in the normal sense of the word... but some of these certainly count as real estate.
Whether I would live in it really depends on whether it has electricity, plumbing and air conditioning...
#1. The world's tallest treehouse is located in Crossville, Tennessee.
Owner and builder Horace Burgess spend 11 years building it. It is 10 stories tall, has 10,000 square feet of space and Burgess began working on it in 1993 after receiving what he says is a vision from god. It only cost him $12,000 to build it since he used mostly recycled materials. The building includes spiral staircases, a basketball course, lots of bedrooms, balconies and a chapel. The structure is supported by not 1, but 6 trees.
#2. The tallest treehouse in British Columbia, Canada.
Its the tallest because of the actual structure is taller.
#3. The 2nd tallest treehouse in British Columbia, Canada.
I decided to give this one 2nd place because it is physically higher up in the tree, 95 feet up in a douglas fir tree, but it is comparatively small and you couldn't really live in it. It is located in Belcarra and was built by George Dyson.
Unfortunately I was unable to find a good quality photo of Dyson's treehouse. The photos that are available it just looks like a tree and you can't see the treehouse that clearly.
#4. 4treehouse near Lake Muskoka, Ontario, Canada.
Built by Lukasz Kos the "4treehouse" is built on four existing trees on the site. The three-story house is suspended from the four primary trees with very little additional support.
#5. The Naha Harbor Diner in Okinawa, Japan
Its big, its a treehouse... but that isn't a real tree. Its concrete in the shape of a tree. The structure even has an elevator and a spiral staircase.
NOTE: This is by no way a factual list of "the world's tallest treehouses". The tallest ones may not even be mentioned on the internet. When you consider that the tree dwellers of the Brazza River Basin in the Indonesian province of Papua all live in treehouses, they probably have quite a few really tall ones.
There is also the "Free Spirit Spheres", which aren't really treehouses. They're constructed on the ground and then suspended in trees using cables. Regardless, they're all interesting on an architectural level.