The hotel alley along Highway 99 near the airport in the town of SeaTac was once a notoriously dirty and sketchy neighborhood, but over the last five years or so the area has really spiffed itself up. Older hotels have been remodeled and decrepit motels torn down in favor of newer establishments. As a result, there are fewer places that remain under $100 per night, but those that do are typically cleaner and safer than what used to be offered in this pool.
One of the best deals in the area is at 
Sea-Tac Crest Motor Inn (18845 International Blvd., 206/433-0999 or 800/554-0300, www.seataccrest.com [1], $69 s or $79 d), two blocks south of the airport. This is a friendly mom-and-pop operation that caters to no-nonsense travelers. Rooms are accessible only by indoor corridor, with 24-hour front desk and security. Beds are reasonably comfortable and the units are spic and span—no need to wear socks in the room here. Coffee and muffins are served in the morning, there’s free shuttle service to the airport (dial #20 from airport courtesy phones) and seven days of free parking are included with one night’s stay.
For something a little slower-paced, but still convenient to the airport, Three Tree Point Bed & Breakfast (17026 33rd Ave. SW, 206/669-7646 or 888/369-7696, www.3treepointbnb.com [2]) rests in the little town of Burien, about 10 minutes southwest of SeaTac. This waterfront inn rests by a little point jutting out into the Puget Sound. There are two light and airy units here. First there’s a deluxe suite ($150 d) with living room, dining room, separate bedroom, and laundry room. And then there’s a cottage ($190) with bedroom and master bath, a beautiful kitchen with gas stove and oven, a living room with fireplace, and a laundry room. Both units come with a porch and some amazing water views. A gourmet breakfast is delivered to your kitchen each morning.
If you've got an early morning flight to catch, Best Western Airport Executel (20717 International Blvd., 206/878-3300, $119 s or d) is one of the best budget chain hotels on the International Boulevard strip. Rooms are very clean, furnishings are new, and access is only by indoor corridor. Get work done in the business center and then relax in the indoor pool, hot tub, and steam room. There’s also a fitness center and a restaurant on-site. Wireless Internet is free, and a hot breakfast is served each morning starting in the wee hours at 5 a.m. The hotel runs a free shuttle to the airport.
Links:
[1] http://www.seataccrest.com
[2] http://www.3treepointbnb.com