Clark County, Nevada
Increase (July 2005 - July 2006): 68,175
In the 1950 census, the total number of Nevada residents just topped 160,000; it was the smallest state in the union.
Now the population of Clark County alone increases by more than that every three years.
The reason: Gaming. The county's main city, Las Vegas, is the heart of the nation's increasingly rabid gambling culture.