I've been fascinated by the Nakagin Tower for years. It was built in 1972 and consists of one hundred and forty steel capsules, each having just over a hundred feet of usable living space, connected to two central concrete towers (which deliver power and other facilities) by just four high tension bolts. Each of the capsules were fully fitted out before shipping, and could be independently replaced without affecting neighboring capsules.
National Geographic has some great interior shots of some of the tiny capsules which some people still call home. The photos in the article are by photographer Noritaka Minami. There are lots more on the artist's site.
I'd love one as a tiny little hideaway.