Alaska Airlines is now selling tickets for flights from Seattle's planned second airport, Paine Field in Everett. The 18 daily flights are still subject to FAA approval, but everyone appears to expect that soon; even the government's own TSA is hiring staff for the field.
If all goes as expected, United will also start offering flights later next year. Southwest had originally said it would, also, but in a change of plans sold its slots to Alaska, which then added more flights.
The airport, which is also home to Boeing's large-plane assembly and delivery operations, has not had scheduled service for many years. Paine is about 25 miles north of Seattle, while busy SeaTac is about 15 miles south of the city. For passengers coming from the north, a flight out of Paine could save a long time in traffic.