We're currently putting together an episode on architect/engineer R. Buckminster Fuller (1895 - 1983) and we can't help but dream about such wonders as his Cloud 9 concept. This entailed an enormous floating geodesic sphere, held aloft in the air due to the massive volume of warmed air inside the otherwise low-mass structure. Fuller proposed that such a sphere could actually support airborne towns.
Interestingly enough, a 1971 edition of the Russian popular-science publication Technica Molodezhi featured a design for a similarly-designed means of floating human habitats in the planet Venus's far-more-tolerable upper atmosphere. The surface of Venus is a pressure cooker, but up there we could theoretically live in such amazing habitats.
You can read more about it at Robert Walker's Science 2.0 blog, as well as the original posting on the Secret Projects message board.