As a Kitsap Peninsula resident, SEA is a bit of a journey for me and I have a major loathing of the parking options. So I've tried to be creative in finding ways to get to the airport without hooking up one of the parking firms on 99.
Recently I tried the 574 Express bus to SEA (via I-5 & Federal Way) from the Tacoma Dome parking garage. I know there are posts about taking the bus from Seattle, but I didn't see any about riding from Tacoma, so here goes:
The bus stops several times so it's not as fast as driving it yourself, but it's $2.50 each way and the buses are comfortable. Not a huge amount of luggage space, so I don't suggest the 574 if you're traveling with big bags during a busy time of the month, but enough for a couple of small carry-ons I should think.
The bus drops you off at the end of the arrivals drive near the Airporter departure area/door 00 and picks up about 100ft further down the drive. You can't make change on the bus but there is a change machine available at the Tacoma bus stop (there are some inside the bag claim level at SEA as well, but they were out of order).
Regarding the parking garage - it's free unless there's an event at the Dome. Technically you can't stay more than 24 hours. I recently parked there for several days, though, and walked to the Amtrak station for a trip to Portland. When I returned, I had been ticketed by the Diamond Parking staff, but not for 24hr+ parking - it was for not parking exactly between the lines, and it took them three days to ticket me (and I was within the lines, dang it). I called the service number on the ticket and got the $25 fee waived (lucky), but it seems to me that garage management doesn't check too rigorously.
Use at your own risk of course, but the 24 hour rule does make it possible to save cash if doing a quick mileage run out of SEA by just taking the 574 for $5.00 r/t instead of paying up for parking around SEA.
Needless to say, you can also just be dropped off at any 574 stop.
My other option is the Holiday Inn Kent, which has an airport shuttle, does not charge for parking, and allows seven days of parking with a one night's stay at the hotel.
I reviewed the hotel here
(shameless plug) in the IC Hotels forum and think it's a great value. Their rates are usually much lower than park & fly rates offered around SEA. I know there are many other hotels in the area that offer a free ride and free parking, but this is the only one I've seen with free park & fly.
Anyway, just trying to do my part to deny money to the parking outfits around SEA.
If I'm flying out of SEA on business, I park at MasterPark and claim it, but if it's on my own dime I'm going to do whatever I can to keep parking from costing more than my airfare.