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. The town of San Pedro here contained an overflow camp that was investigated by Torchwood. (TW: The Categories of Life)

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 end of the 21st century. (DW: Doctor Who)

Before joining the CIA, Rex Matheson lived in Los Angeles. After the CIA turned against him, he later returned here, working with Torchwood to investigate PhiCorp. (TW: Rendition, Escape to LA)

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 tales, PDA: The King of Terror; and TW: Escape to LA and The Categories of Life.