I'd like to add a bit more info on the possible geographic origin of hella (and defend the honor of my hometown of San Francisco). When I attended UC Santa Cruz from 1995-1997, it was clear to me and my friends that the term originated or at least found an early foothold in the Central Valley - of the four people we knew who used the term regularly, two were from rural Central Valley areas and one was from Sacramento (the largest city in the valley). Since then it has taken hold in the mouths of junior high school girls and, from there, the masses.

It should also be noted, contrary to Inyo's w/u, it is used in the Los Angeles area, albeit mostly (in my experience) by children.