Seattle's new light rail service can be used to get you from seatac to downtown seattle for around $10. Make sure you have at least a 3hr layover as it took me about 45 minutes each way. Beats the heck out of waiting around at seatac forever though.
Yes it take 45 min to get to the airport on the light rail but you save gas, $, not to mention the ENVIRONMENT!! If we all consistently took the light rail to the airport, our dependance on cars and oil would drastically change. GO LIGHT RAIL!
Seattle traffic blows. The light rail from Seatac is a godsend. I live in Des Moines and would park in the park and ride from the stop just north of seatac to ride the light rail to Seahawk games. Best way to commute to downtown ever.
Even though the light rail is kinda slow, I dig it. It's clean, quiet and it's a pretty nice route. The busses are faster but I've had way less strange/weird/terrifying/disgusting experiences on the light rail than I have the busses.
I paid approx $2.25 one way to downtown. On the plus side, if there are traffic issues on either I-5 or I-405, you'll save time.
Unless you live right on the light rail line, you may have to employ a second mode of transport (no parking at the actual light rail stations!) which kinda defeats the positive environmental impact. Currently, ridership hasn't matched up to expectation, but that may change over time.