These are all landside. For sleeping options airside, see previous post.
(As an example of what I consider walkable, at Boston Logan Airport I count the Hilton Boston Logan walkable from the terminals, while the Embassy Suites Boston-Logan and the Hyatt Harborside I don't. The Hilton Boston Logan is about 500 meters from Terminals B and C, and less than 400 from Terminals A and D.)
In some cases, like Heathrow, there's a hotel listed which is attached to one terminal but that terminal isn't particularly close to other terminals. Often there's some free method of inter-terminal transportation you should take if you're at some other terminal.
None of this should be construed as a recommendation for or against any of these hotels; in general, they're expensive and inconvenient to the city they serve. They are, however, convenient if you're stuck at the airport. These are the hotels you should probably try for first if your airline is responsible for your lodging.
The fifty busiest airports (2009), in order :
Hartsfield–Jackson Atlanta International (ATL): Atlanta Airport Marriott Gateway
SpringHill Suites Atlanta Airport Gateway (both walking distance from the Gateway Center station of the ATL Skytrain)
London Heathrow (LHR): YOTEL™ Heathrow (mezzanine level, Terminal 4) Hilton London Heathrow Airport (connected by covered walkway to Terminal 4) Sofitel London Heathrow (connected by covered walkway to Terminal 5)
Beijing Capital International (PEK): Pay-in Lounge, (premium paid lounges with sleeping areas and rooms; locations in each terminal)
In theory, there is also the Beijing Airport Huiqiang Hotel (北京空港惠强宾馆) within walking distance of Terminals 1 and 2, but I have very few details as it doesn't seem to be coming up in Chinese hotel booking engines.
The Langham Place Beijing Capital Airport Hotel (北京首都机场朗豪酒店) is the closest hotel to Terminal 3 but is more than 600m as the crow flies from the terminal entrance. The walk is probably more like a km, because you can't walk straight there. You'll need their shuttle.
Chicago O'Hare International (ORD): Hilton Chicago O'Hare Airport (within Terminal 2)
Tokyo International (HND) (despite the name, this airport is primarily a domestic airport. It is also known as Haneda Airport, and should not be confused with the much more distant, primarily international Narita Airport (NRT), listed later): Haneda Excel Hotel Tokyu (羽田エクセルホテル東急) (connected to Terminal 2)
Paris-Charles de Gaulle (CDG): Sheraton Paris Airport Hotel (inside Terminal 2) (Note: CDG has a number of other hotels on the airport grounds, all of which are too far to walk to from a terminal.)
Los Angeles International (LAX): The Radisson Hotel at Los Angeles Airport is the closest hotel to the airport, the first block outside the airport grounds on West Century Boulevard. Next door is the Courtyard Los Angeles LAX/Century Boulevard. That having been said you'll still probably end up taking a hotel shuttle as the intersection where Century crosses Sepulveda and goes into the airport is not the easiest to negotiate on foot with luggage. Moreover, the airport stretches away from the entrance, so unless you're going to or from one of the closer terminals (1, 7, or 8) you won't really be walking distance anyway.
Dallas-Fort Worth International (DFW): Grand Hyatt DFW (inside International Terminal D) Hyatt Regency DFW (on the other side of the Terminal C parking garage)
Frankfurt (FRA): Sheraton Frankfurt Hotel & Towers (connected by pedestrian bridge to Terminal 1)
Denver International (DEN):
New York John F. Kennedy International (JFK): There is a Ramada Plaza on the airport grounds, but it is not walking distance from the terminals.
Hong Kong International (HKG): Regal Airport Hotel Hong Kong (connected by air-conditioned bridge to Terminal 1)
Amsterdam Schiphol (AMS): Sheraton Amsterdam Airport Hotel (connected by pedestrian bridge to terminal) (other airport hotels which are farther away but also within walking distance: the Hilton Amsterdam Airport Schiphol and the citizenM Amsterdam Airport)
Dubai International (DXB): There are some hotels quite close to Terminals 1 and 3 but there doesn't seem to be any obvious way to walk to them as Airport Road blocks the way. Le Meridien Dubai, Al Bustan Rotana (فندق البستان روتانا), and the Millennium Airport Hotel Dubai (فندق ميلينيوم المطار) are the closest to Terminal 1, while the Holiday Inn Express Dubai Airports and Premier Inn Dubai Airport are closest to Terminal 3. There's nothing much close to Terminal 2.
Bangkok Suvarnabhumi (BKK): Novotel Bangkok Suvarnabhumi Airport (across parking lot from terminal)
Las Vegas McCarran International (LAS):
Houston George Bush Intercontinental (IAH): Houston Airport Marriott at George Bush Intercontinental (in center of airport between Terminals B and C, connected to all terminals by Inter-terminal train)
Phoenix Sky Harbor (PHX):
San Francisco International (SFO):
Singapore Changi (SIN): Crowne Plaza Changi Airport (connected by covered walkway to Terminal 3)
Jakarta Soekarno-Hatta International (CGK): Jakarta Airport Hotel (in the terminal building on the second floor of Meeting Point Terminal 2E)
Guangzhou Baiyun International (CAN): Hotel Pullman Guangzhou Baiyun Airport (广州白云机场铂尔曼大酒店) (3 minute walk to terminal)
Charlotte Douglas International (CLT):
Miami International (MIA): Miami International Airport Hotel (second floor of Miami International Airport in Central Terminal Concourse E)
Rome Leonardo da Vinci (FCO): Hilton Rome Airport (connected by walkway to terminals)
Orlando International (MCO): Hyatt Regency Orlando International Airport (within the airport terminal on Level 4)
Sydney Kingsford Smith (SYD): Nothing near the International Terminal. The Stamford Plaza Sydney Airport is just outside the airport on the corner of O'Riordan & Robey Streets. The Hotel Formule 1 Sydney Airport is a slightly longer walk from the Domestic terminals.
Newark Liberty International (EWR): Newark Liberty International Airport Marriott (across a parking lot facing the terminals)
Munich (MUC): Kempinski Hotel Airport Munich (between the two airport terminals, adjacent to Terminal 2)
London Gatwick (LGW): YOTEL™ Gatwick (lower level, International Arrivals, South Terminal)
Sofitel London Gatwick (connected to North Terminal by covered walkway)
Hilton Gatwick Airport (connected to South Terminal by covered walkway)
Minneapolis-Saint Paul International (MSP):
Tokyo Narita International (NRT): Narita Airport Rest House (成田エアポートレストハウス) (between Terminals 1 and 2)
Shanghai Pudong International (PVG): Motel 168 Shanghai Pudong Airport (莫泰168上海浦东机场空港宾馆) [Chinese only, as the English website at http://www.motel168.com/En lacks useful information] (above the Maglev station between Terminals 1 and 2) There's also a Ramada nearby but it's not within a half km of the terminals.
Detroit Metropolitan Wayne County (DTW): Westin Detroit Metropolitan Airport (inside the McNamara terminal)
Seattle-Tacoma International (SEA): Hilton Seattle Airport Hotel & Conference Center (directly across International Boulevard from the terminal, on the other side of the parking garage) There are a number of other hotels along International Blvd., including a Holiday Inn, a Radisson, a Red Lion, a Concourse Inn, and the Jet Motel, but the Hilton is the closest. You can avoid crossing International Blvd. by using the Sound Transit light rail pedestrian bridges.
Philadelphia International (PHL): Philadelphia Airport Marriott (skybridge to Terminal B)
Toronto Pearson International (YYZ): Sheraton Gateway Hotel in Toronto International Airport (direct indoor access to Terminal 3 via bridge)
Istanbul Atatürk International (IST): Istanbul International Airport Hotel (adjacent to International Terminal)
Kuala Lumpur International (KUL): Pan Pacific Kuala Lumpur International Airport Hotel (connected by skybridge to Main Terminal) Tune Hotels KLIA-LCCT Airport (around parking lot from Low Cost Carrier Terminal building)
Seoul Incheon International (ICN): Incheon has some hotels on the airport grounds, none of which look like an easy walk from the terminal. The Hyatt Regency Incheon (하얏트리젠시인천호텔), Best Western Premier Incheon Airport (베스트웨스턴호텔 인천에어포트), and the Incheon Airport Guest House (인천공항게스트하우스) are all over half a mile from the terminal.
On the other hand, following a tip from cityratbuddy--thanks!--I find there's a Jjimjilbang (Korean public bathhouse) called Spa on Air on the basement level of the main terminal, which also provides private rooms as an add-on to spa admission.
Barcelona (BCN): Air Rooms, part of the AirMeeting Business Centre (part of the AirMeeting business centre (centro de negocios) in Terminal 1, First Floor)
Logan International (BOS): Hilton Boston Logan Airport (connected to Central Parking garage by skybridge, easiest from Terminals A and E)
New Delhi Indira Gandhi International (DEL): Airport Hotel: (across a parking lot from Terminal 1C), reviews--not particularly good--at tripadvisor.com Note: Terminals 1C and 1D are not close to Terminal 3. Budget an hour to transit between them.
Melbourne (MEL): Hilton Melbourne International Airport (connected by covered walkway to terminal) Holiday Inn Melbourne Airport Formule1 Melbourne Airport (both across parking lot and roadway from terminal)
Paris-Orly (ORY): Hilton Paris Orly Airport (between the west and south terminals) Ibis Orly Aéroport (northeast of the south terminal)
Not an easy walk, but within about 500 meters of the terminal as the crow flies. Your route will not so direct, whether you attempt to walk or take the shuttle.
Shanghai Hongqiao International (SHA): Shanghai International Airport Hotel (上海国際機場賓館) (across parking lot from entrance to Terminal 1)
Mumbai Chhatrapati Shivaji International (BOM): No hotels within a short walk of the International Terminal. There are a number of hotels within walking distance of the Domestic Terminals: Hotel Sahara Star (across parking lots from Terminals 1-A and 1-B)
Farther away behind the Sahara Star there are a number of hotels including the Atithi, Transit, and Orchid hotels. On the other side of the Western Express Highway but still close to the terminal are the Avion and Aircraft International hotels.
Shenzhen Bao'an International (SZX): Nothing except the Airport Hotel 机场宾馆 (just within a half kilometer of the terminal, to the right along Airport 4th Road [机场四路]), which I have been unable to find a trace of on the web, which is a bad sign. Usually there are some reviews on Chinese hotel booking sites, but there's nothing for this one.
Other hotels in that direction include the Civil Aviation Hotel (民航酒店), but it's another 300 meters down the road.
Mexico City International (MEX): Hilton Mexico City Airport (Terminal 1, 3rd Level International Arrivals area, Door 8) Camino Real Aeropuerto Mexico (connected by pedestrian bridge to Terminal 1, Gate B) NH Aeropuerto T2 México (Terminal 2, 5th Floor)
Airports without a hotel link after them have no hotel within walking distance that I was able to find.
I put together this list for the convenience of people who may find themselves stuck at one of these airports during the holiday season, and may or may not be updated if I feel like it. All information correct when viewed 11 November 2010 and subject to change. If you know of a hotel I haven't listed at an airport I have, or if you find any errors, please let me know.
19 Feb 2011: More airports. Also added hotels at LAX, SYD, SEA, ORY, and BOM.
11 Apr 2011: Added info about the Langham Place Hotel near Terminal 3 of Beijing Capital Airport.