Ours were always made out of leftover lumber we scavenged, and they usually had rusty nails sticking out of them. But they were great! And even though my brothers didn't always let me actually come inside the treehouse, just knowing it was there was pretty great.
Now feast your eyes on this piece of heaven:
I want to go there. (30 Rock, anyone? anyone?)
I want to sell my house and live like an ewok. Is that so wrong?
Here are some more awesome - and a tad ridiculous - treehouses:
Oh tree houses. I want to climb up and live in you. I like you.