I heard not too long after moving to Portland that among all of the fabulous parks in the city, we had the World's Smallest Park. The story behind this park just goes to show you how original and quirky the people that live in this city can be, and part of why I love it.
Mill Ends Park is 24" in diameter and was acquired by the Portland's Park & Rec in 1976 it is in the center meridian at the intersection of Taylor Rd and Naito Pkwy (Front Ave). Originally this park was supposed to house a light pole, but when the light pole never came, the park's founder Donald Fagan decided to pretty up the useless weed filled dirt spot and planted flowers in it.
Read more about it here: http://www.portlandonline.com/parks/finder/index.cfm?PropertyID=265&action=ViewPark