As I travel more for business and fun, I'm slowly realizing which airports have good access to town, either on foot or by transit. I thought it would be nice for us to build up a list of airport-transit hacks for each city we know... the money you save on a taxi or shuttle can pay for an extra hostel night or a meal!
I'll start...
Toronto (terminal 1): all the way downstairs and to the far right, then out the doors for the TTC's #900 express bus (the "Rocket") to the Kipling subway station. $3.25, no transfer needed unless you want to take another bus at the far end of your subway trip. Late at night, take the #300 night bus instead (it runs all the way east across the city). Walking's not on... the access roads have no sidewalks.
San Diego: This one rocks! The 992 bus takes you downtown in about 10 minutes. If you're heading in the other direction (toward Point Loma or Ocean Beach), walk out to Harbor Dr. and take the #923. You can also take the free parking lot shuttle that lets you off at the Middletown tram station, which is better if you want to buy a transit pass. Walking is easy from this airport -- downtown along Harbor Dr. (mostly along waterfront paths) takes half an hour to an hour, depending on where you're going. In the other direction, it's about an hour to Point Loma.
Montreal: Take the "747" express bus to the Lionel-Groulx subway station (25-30 minutes). $10 includes a 24-hour transit pass. You could also walk to the Dorval city bus terminus (about half an hour) and take a slower city bus for about three bucks. Walking elsewhere, unless you actually want to be in Dorval, is not really an option.
St. John's (NL): Mon-Sat between about 8 am and 11 pm, the #14 city bus ($2.25) stops at the airport (out the door, turn right and follow the sidewalk). It's about a half hour meander to the university, where you can transfer. Or, you can walk into town along Portugal Cove Road (turn left out the door, follow the sidewalk to a gap in the fence to Airport Rd.).
Vancouver: Skytrain! Fare depends on distance travelled, and the ticket vending machines at the airport add a $5 airport surcharge. You can apparently avoid the surcharge by buying a day pass ($9.75) from the 7-11 or Pharmasave on the arrivals level. Walking: I know I've walked in the past, to a bus stop, and it looks like you still can, along Sea Island Way, but I don't see the bus stop. I guess you could walk to the Richmond Night Market, Fri-Sun in summer.
Los Angeles (LAX): take the G shuttle (stops in front of each terminal) to the Aviation/LAX stop on the Metro green line; from there, the green and blue line (or harbor fwy express bus) will take you downtown (note that you're expected to pay a fare for each separate vehicle you ride, so a day pass is a good idea). If you're going to Venice or Santa Monica, take the C shuttle to the LAX bus transit centre (ask the driver) and catch the #3 or #R3 Big Blue Bus. To Manhattan Beach, the #232 from the same transit centre.
Walking? I don't know if you can, or where you would go. If you do want to walk, I would take the C or G shuttle anyway, and walk from there.
London Heathrow: From terminals 1, 2, and 3, you walk through a surprisingly long tunnel to the Tube station (terminals 4 and 5 apparently have their own stations). Fare into town is 5 pound 50. If you're going to be there long or often, buy an Oyster Card, which will give you much cheaper rides for the rest of your trip (and magically convert itself into a day pass if it's to your advantage). I've never walked anywhere from Heathrow.
Baltimore: Another excellent transit connection. Go to the far right hand end of the terminal, out the door, and you're almost on the platform for the Light Rail. Super cheap: $1.60 for a one-way, $3.50 for a day pass good on the bus and metro, too. You can also take transit into DC from here, but it's more of a MacGyver: M-F you can take a free shuttle to the MARC train, on the weekend you take the B30 (scenic!) to the Greenbelt metro station and thus into town.
I know a few more, but I should stop now, especially as some of you probably know them better than I do.
(edited June 10, 2019, to reflect changes in San Diego, Toronto, and St. John's)