There are two types of floating cities: cities that float in water (see ocean colonization) and cities that float in the sky (such as Cloud City). Both are still theoretical; the latter is really theoretical.
Geoffrey Landis provides an interesting point though about building floating cities on Venus, whose surface is generally considered to be akin Hell:
However, viewed in a different way, the problem with Venus is merely that the ground level is too far below the one atmosphere level. At cloud-top level, Venus is the paradise planet.
(quote from Wikipedia)