SEA has a one terminal, four concourse structure with a pair of satellite concourses. It's runways, 16/34 (L&R), are the only two runways there, and as they are situated side-by-side instead of on either side of the terminal, all the action is concentrated into one place (making life easier for spotters). Currently the airport is constructing a third runway to help reduce arrival delays caused by bad weather, and the central concourse is about to begin major renovation (although I think it looks really good already). Northwest and Alaska make up a large chunk of the traffic here, and SEA is one of only three U.S. airports serviced by Scandanavian Airlines. All concourses are linked by an underground subway system that allows for you to quickly and painlessly shoot from concourse to concourse.
INDOORS:
-Concourse B, Gate B9. Great view of post-rotation departures on 16.
-South Satellite Concourse, Gate S9. Perhaps the best view of 16 departures in the entire airport. Take the train to the satellite, the go to S9, and sit back and watch a powerful view of post-rotation takeoffs.
-North Satellite Concourse, Gate N13. Good view of departures on 34 or arrivals on 16. Best view of the threshold of 16...from here you can watch planes taxi all the way down to the end and, if they have to, wait in line for departure.
-Concourse C, Gate C14. Perhaps the best place (maybe even the only place) to catch rotations on either runway. View can often be obstructed by parked commuter prop-jobs at nearby gates.
OUTDOORS:
-Viewpoint Park Spotters' Lot. Situated on the west side of the airport off Starling Dr. there is a spotters' lot with plenty of parking and a great view of the airport. The fence is tough to try and see over (I solved the problem by climbing up a tree, which security probably wouldn't have liked), but the view is really great here. Unfortunately, this lot will soon be paved over (or has been paved over already) to make room for the third runway. It's a real shame, because the Pacific Northwest will have lost its single best spotting area.
-S. 156th Wy. This street runs along the north side of the airport below the final approach for 16. On days when planes are arriving from the north, they fly right over your head at low altitudes on this spot. Park your car in a nearby lot and watch the planes buzz your head.
PRE-9/11 SECURITY REPORT:
-Security maintains a strong presence at SEA but seems to be very friendly. The Starling Dr. lot is regularly patrolled by almost every police and security force in the city, but all they usually do is drive in and drive back out again without stopping. Airport Authority regularly drives along the inside perimeter fence, but they never seemed to mind me and my camera. You shouldn't have too big a problem with security at SEA. 
