California

California was a state of the United States of America. Its major cities included Los Angeles and San Francisco. During the 20th century it was the location of the Mount Palomar Observatory; in the far future, the Los Angeles Crevasse would (presumably) be found here.

New California was named for this state. (PDA: Synthespians™)

Notable residents
Peri Brown was resident in California at the time of her trip to Lanzarote, at least according to the unofficial, handwritten information at the beginning of her passport. (DW: Planet of Fire) It was likely, however, that this was a temporary address, as her family was from Baltimore, Maryland. (PDA: Blue Box, BFA: The Reaping)

More definitely, Dr. Grace Holloway was a homeowner and practising heart surgeon in San Francisco at the dawn of the 21st century. (DW: Doctor Who)

Behind the scenes
Several stories have been set in California, including DW: Doctor Who, EDA: Vampire Science, Unnatural History; and TN: Wonderland — all set in San Fransisco — and the Los Angeles-based tale, PDA: The King of Terror.