Dutch architects zecc architecten converted a water tower into a nautilus-shaped vertical luxury flat that looks surprisingly homey. I'm especially fond of the little sundeck on the roof.
(Thanks, Justin!)
Update: Katie sez, "An episode of grand designs, the UK self-build design-porn show, featured a Lutyens-designed water tower that a couple had in their garden. They decided to sell their house and build a new one inside the stilts of the water tower."
Update 2: Julian sez, "The girlfriend of a close friend of mine designed and converted this.
I went there for a party and found that while it is indeed, amazing. It's also hell on the legs and would certainly keep you fit."