San Francisco International Airport
County: San Mateo County
Airport code: SFO
San Francisco is a major international airport with direct service to major cities around the world. It is served by major US and foreign airlines. The airport has three major domestic terminals (1, 2, and 3) and one international terminal. (airline information)
An automated train system called AirTrain provides free transportation between the terminals, the BART station at the airport, some parking garages, and the Rental Car Center. AirTrain runs every few minutes 24 hours a day.
BART trains stop at the San Francisco International Airport Station, which is located by the International Terminal. The BART system serves Downtown San Francisco and the East Bay and trains generally operate every 15 to 20 minutes 7 days a week.
SFO employees are eligible for a fare discount on BART. 
Caltrain, which serves San Jose, stops a mile away from the airport at Millbrae. BART provides connection from SFO to Millbrae Station. During nights and weekends, BART provides direct trains between SFO and Millbrae. During daytime, riders need to transfer between BART trains at San Bruno Station.
BART provides connection between Amtrak trains at Richmond Station and SFO.
Bus - SamTrans
SamTrans provides low cost local and express bus service.
140 Pacific Manor - SFO Rental Car AirTrain Station via San Bruno BART
292 Hillsdale Station - San Francisco (Transbay Terminal)
397 All-Nighter: Palo Alto Station - San Francisco (Transbay Terminal)
398 Redwood City Station - San Bruno BART via SFO
KX Redwood City Station - San Francisco (Transbay Terminal)
Line 292/397 is the cheapest form of transportation to/from San Francisco. All SamTrans buses run on the lower level of the terminals, serving the same stops. See the headsign for bus destination. KX has restrictions on carrying luggage between downtown SF and airport.
Airporters provide long distance bus service. Advance reservation is required for some companies.
Evans Transportation Napa/Vallejo - SFO, Napa/Vallejo - OAK
Marin Airporter Marin County - SFO
Monterey Airbus Monterey - SJC - SFO
Santa Cruz Airport Flyer Santa Cruz - SJC - SFO
Sonoma County Airport Express Santa Rosa/Petaluma - SFO, Santa Rosa/Petaluma/San Rafael - OAK
Bicyclist are prohibited from riding on the main roadway at the airport, but are allowed on McDonnell Road, North Link Road, and South Link Road.
Bicycle parking is available on the ground level by the International Terminal and inside Domestic Garage (valet parking) level 4, section C. Bicycles can be parked for up to 14 days by registering with an on-duty security guard or valet agent. Registration is not needed for parking less than 24 hours.