You mean as a way of inserting toops or as a method of resupplying besiged forces?

In Septermber of 1944, The Allies tried airdropping troops via parachute and glider directly into the city of Arnhem as part of Operation Market Garden. Briefly, this was a operational disaster, and the mostly-British allied forces lost pretty badly.

There have been other instances of resupplying besieged troops via air, most recently during the Seige of Sarajvo in 1992-1996, NATO airdropped some 60,000 tons of food and other humanitarian supplies to prevent starvation of the civilian population, as part of Operation Provide Promise.