I think we were having this discussion somewhere on FlyerTalk. There are two buses that go between the airport and downtown. The folks on FlyerTalk suggested taking the express bus, since it takes half as long to transit between the two. Here is some info copied from Wikitravel:
* Metro(city bus) - Routes 194 (express, 30 minutes) and 174 (45-60 minutes) will get you downtown for $2.25 during peak hours (6-9am and 3-6pm) and $1.50 at all other times. Get exact change by buying a latte at the little Chinook coffee stand by the baggage claim downstairs, then exit the terminal, turn right and walk all the way to the south end of the building where you will find a couple of Metro bus stops with schedules posted. Be aware that the 174/194 at the first bus stop head downtown; the 174/194 at the second (adjacent) bus stop head out to Federal Way.
* Taxi - The trip is about 20 minutes by taxi (expect to pay $30-40 plus tip); catch one on the third floor of the parking garage.
* Rental car - On a weekend, you might want to shop the internet for rental cars, since they can be less than $12/day (plus roughly 18% tax; also consider hotel parking fees, if any). Beware of the fact that taking a rental from the airport will incur an 11% "airport tax" surcharge. If you are able to rent a car from a downtown location you will not have to pay this and will save a considerable amount of money.
* Visitor Center - If you come through the airport but still need help planning your trip, making hotel reservations, tour bookings, getting area and downtown maps and more, make sure you stop by the Seattle Southside Visitor Centerat exit 156 off of Interstate 5. It is the official visitor center for the SeaTac International Airport and Seattle Southside region.
* Commercial shuttle buses are about $5-$12.75 and probably not faster than the bus if you are going downtown.
* Also a special note. In the fall of 2009 Sound Transit will begin operating a light-rail line between downtown Seattle and the Seattle-Tacoma (Sea-Tac) International Airport. (
http://www.soundtransit.org/x1171.xml)