Although it is not a major international aviation hub, Berlin's history has blessed it with three airports:

Berlin's Tegel Airport (TXL) - front entrance area Tegel (TXL) is Berlin's busiest airport and is served by most major European airlines. It is quite close to the centre of western Berlin and is the main airport for domestic flights. However, it is due to be closed in 2012.
Schönefeld (SXF) is Berlin's "bargain basement" airport and is served by low-budget / no frills and charter airlines. It is located just outside Berlin, and is currently being expanded to be the city's sole airport.
Tempelhof Airport front entrance Tempelhof (THF) was Berlin's oldest and smallest airport, served mainly by regional carriers. Now closed, it is located fairly close to the centre of Berlin.

Tegel and Tempelhof are located within the former West Berlin, while Schönefeld was East Berlin's international airport. The city is planning to consolidate all air traffic at Schönefeld, which is being rebuilt as Berlin-Brandenburg International Airport (BBI). Tempelhof, the smallest of the three airports, and in the middle of a residential zone, was finally closed on October 2008. Tegel, which also lies well within the Berlin city limits, will be closed when the new airport is completed - the plans say in 2011, but big projects are inevitably subject to delays, and Berliners are putting their money on 2013 or 2014.