Traditional Taxi Cabs 

Cab fares from Sea-Tac International Airport to downtown Seattle run $42-50 one-way, and are available through Yellow Cab.  Once downtown you can take your pick of cab companies. The best way to find one is to have the hotel or restaurant call one for you if none are waiting nearby.  During peak hours and weekends you may wait 10-15 minutes before getting one to stop or drive by.  While cabs are metered from the airport to downtown, cabs charge a flat fee (currently $40) from the downtown hotel zone to Sea-Tac Airport.  Sea-Tac Airport has an agreement with one taxi firm who's cabs are stationed in the Sea-Tac garage.  That firm is currently Seattle Yellow Cab.  STILA is the on site limo and town car provider at Sea-Tac established by the Port of Seattle. Since they charge fixed rates, the amount for a limo into Seattle is often as cheap as a cab.  Town cars and limos from the airport are available by calling #55 from the phone near the hotel ads in baggage claim. The current rates are $45 for a town car. See also:  Sea-Tac Airport Ground Transportation and  Taxi Fare Estimator.

Links: 

Farwest Taxi 

Orange Cab

Seattle Yellow Cab 

 

Transportation Network Operators (e.g. Uber or Lyft)

New app-based "ride-share" services make hailing a ride to/from the airport easy from your smartphone. UberBlack offers flat rates, with no tipping from the airport to downtown Seattle at $55 for a towncar. Within the city of Seattle, rides via uber are less expensive than traditional cabs and more convenient. Uber offers several levels of service, which "UberX" being the least expensive. There is no expectation of tip and your credit card is charged through the app. More information here:  Uber Ride Sharing App  

The same is true for Lyft, although their app does not provide an estimate of how much the fare would be. An estimated ride from the airport would be about $30 based on distance and a 30 minute drive time (may vary, as with cabs), and within the city a ride should be less than a taxi.  As with Uber, there is no expectation of tip and your credit card is charged through the app. More information here:  Lyft Ride Sharing App 

Both Uber and Lyft are available for trips anywhere in the city and in practice will result in a car picking you up with 15 minutes in most places. The new application based service also has several advantages that traditional cabs do not provide such as being able to see where your car is before it arrives, your exact route, the ability to rate your driver, and no disputes about money or rates (and no tipping required or expected). 

"Flat Rate" Taxi Cabs

Seattle offers a flat rate based on zip code that is not metered by time or mileage. You must pre-book these trips. They currently offer a rate from downtown to Sea-Tac for $29,  . This company offers ride in the city, as well as to and from the airport. One of the largest of these firms is Flat Rate for Hire. They have 400 cars which cover the Seattle metro area.  You just call or text them when you need a cab, but they are not allowed to pick up ad hoc fares inside the City of Seattle.

There is a tool for estimating fares available online, which gives you the ability to compare taxi fares.  The site lets you plug in an origin and destination, then comes up with a fare which doesn't include any airport fees. In general the for hire vehicles almost always are cheaper than a metered cab. The advantages multiply if you arrive in rush hour or during heavy traffic. Wait time in cabs are charged at $.50 a minute, whereas for hire vehicles are always a flat rate.

The ''official' airport taxi fare estimator shows an average fare from Sea-Tac to downtown Seattle at $52.50 making the fixed rate STILA limos a great value. 

You can check them out here:  flatrateforhire.com