The transit zone of Frankfurt Airport’s Terminal 1 now offers a roof terrace. Besides fresh air, it boasts a spectacular view of the bustling activity at Germany’s largest aviation hub: ground handling, takeoffs and landings.

Covering an area of 138 square meters, the Open Air Deck is well worth a visit. It can be accessed by all passengers in the non-Schengen zone.

Visitors to the airport can also enjoy the panoramic view of the apron and airfield from the Visitors’ Terrace (see photo below), which is located on Level 4 (next to the Food Plaza) in the publicly accessible part of Terminal 2. Details at the Frankfurt Airport website

Terminal 2
From Wondertree in the departure lounge there are great views out over the runway.

Terminal 3
There are panoramic runway views from the No.1 Traveller Lounge (a charge applies), and more intimate views from some of the departure gates.

Terminal 4
You'll find our View Heathrow observation deck (near Gates 15 and 16) the perfect vantage point, with its 270-degree panoramic view of the southern runway, binoculars and flight-tracking touch screens. (see photo below)

Terminal 5
Once you're in the departure lounge there are airfield views almost everywhere, but Gordon Ramsay Plane Food and wagamama offer particularly fine ones.

Heathrow Academy
There's a covered viewing stand for non-passengers at the Heathrow Academy building (formerly the visitor centre) in Newall Road, looking on to the northern runway (postcode TW6 2RQ). There is a charge for parking but the stand itself is free to use at all times.

Details about spectator areas at the Heathrow Airport website

Vilnius Aiport in Lithuania has opened a new plane spooting area where you can watch and photograph aircraft. A convenient location for the site, which is raised above the ground and also be accessible to the disabled, is set up next to the Salininkai Gardens, right behind the fence of Vilnius Airport, where everyone can enter. The location of the site was chosen considering both the technical capabilities and the best view for its visitors as airplanes ascend and descend very close to the site.

The site consists of two parts: an underground and an elevated part specially adapted for the needs of photographers; yet, it is also good for simply watching the aircrafts. A shelter and benches have been set up in the site, and next to it is a car parking lot. 
Details at the Vilnius Airport website

The SkyTerrace is an outdoor observation deck at San Franisco International Airport in California.  Located pre-security in Terminal 2, The SkyTerrace is open seven days a week from 7 AM – 10 PM. Access is free and no ticket or boarding pass is required, but visitors will undergo a security check as they enter the space.  Details at the SFO Airport website