Recently, we stayed at Fairfield Inn close to Seatac airport. We did this thinking it would be convenient for us to drive to CenturyLink for a Seahawk game. What we didn't plan on was the gal at the front desk suggesting we take a shuttle (which they offer for free) to Seatac Airport and take the Light Rail ($2.75) to the game. This saved us a ton of money and headache when trying to find a place to park. We would have never knows this option existed had she not mentioned it. Thank you for our stay that much more comfortable and convenient.
The light rail system is really a fantastic way to get in and out of Seattle from the airport. I used it both ways on my trip there last month and it was so easy! The BART system in San Fran/Oakland is the same.....very easy to use and get downtown from the airport. It's just a litle more expensive ($8.75) each way.
In the NYC/NJ airports have the Airlinks but they are slightly more expensive ~$15 (plus or minus a dollar or two). PHL is directly accessible from the regional transit system but requires a transfer.
London was ~5 GBP (~ 8 USD)
Really great way to travel (faster than ground traffic, and much less expensive) IF you do not load yourself down with too much luggage. And of course you save time and money on the plane if you can pare down to a carry on.