The Uros people of Lake Titicaca along the border of Bolivia and Peru live on man-made floating islands constructed of reeds! The group developed this system to protect themselves from invaders, and now it protects their culture from unwanted contact and assimilation. The thick layers of reeds must be replenished several times a year. Link -Thanks, RJ Evans!
(image credit: Flickr user Yan-Di Chang)
Everyone there tends to agree (if you ask them off the record): The islands are run now strictly for the touristic value, and are mostly just a spot to put the souvenir stands on. You also won't find many old people there. Those are enjoying the dryness and the comfort of the main land too much.
The island are a wonderful sight, nonetheless. Interesting is also that the new "Uros" have a valuable environmental function: They keep the Titicaca lake clean of plastic bottles, as they discovered they can be used as effective floating bodies hidden inside their reed-boats. :-) That is a relatively new technology, as you might guess.