Note: in many cases there are shorter walks available to either nearby public transport that may be cheaper than the more expensive trains to the airport, or to nearby attractions. If you edit this wiki, please post those as well.

Airport - Notes (include time and a link to a google map route)
AGP - approx. 40 minutes from Holiday Inn Express, which is the nearest airport hotel
ANC - downtown 1h 20'
BOM - from domestic terminal to Ville Parle railway station (crowded on week-days, OK on week-ends) (20 mn)
BRE - center (45 min)
BRU - from Zaventem train station
CNX (Chiang Mai, Thailand) - 48 minutes from the far side of old town, less to near side
CPH - city center(1h30 min)
DAD (Da Nang, Vietnam) - one hour to the beach, 40 minutes to the center
DAL - downtown(2h) or take bus 524 for free to Inwood Station and then walk. Costs $2.50 TO DAL.
DCA - 15 minutes. Crystal City, Arlington.
EDI - short walk to the nearby Hilton. Airport bus also stops across the street from Hilton and goes to centre of town
ETH - AIRPORT CLOSED
FUK - 45 minutes to Hakata Station
GIB - at the north end of Gibraltar. Walkable to La Linea or to Gibraltar
GVA - center (1hr), CERN (1h4),
IEV - City center(1h30 min)
ITM - to Hotarugaike station (Hankyu network) avoiding Monorail (18 mn)
JFK - Lefferts Blvd airtrain station to subway (to avoid airtrain fee) (20 min)
LCA - about 1h to center seafront
LCY - to Canary Wharf (1 hr)
LIS - Oriente (40 mins), city center (1h20m)
LIN - center (1h30m)
LYR - center (1hr) - watch out for the polar bears! (gun required, or make sure you are traveling with someone you can outrun)
MAA - to Tirusulam station across the elevated highway (10 mn)
MAD - T123Barajas metro station (to save on airport surcharge) (30 min)
MEX - T2 to Pantitlan Metro (10 min)
NCE - Arrivals level to the city center's Gare-de-Ville station (75 min)
PEK - Buy a metro card at the metro station but don't take the extremely expensive CNY 25 train there. Walk outside and take 850快 via expressway to the city, CNY 2 only after discount.
PRG - center This map is somewhat deceptive - if you try to walk along the road, there is no room (3h)
SAN - downtown along the coast (40 mins), sidewalks
SDU - downtown Rio (30 min)
SDV - AIRPORT CLOSED
SJC - downtown (1hr)
SJJ - to the nearest trolley bus line (15 min), to the center (2 hrs)
SYD - int'l terminal to Wolli Creek Station (to avoid the airport surcharge) (20 min)
domestic terminal to Mascot station (to avoid the airport surcharge) (20 min)
TLS - to the center (1h40m)
TOS - center (1hr)
TSA - to center (40 mn to 1 hr)
WAW - center (1h40m)
WIC - to the centre (15 min)
WLG - center (1h30m)
YTZ - tunnel to the island is now open.