A prize won merely by entering a place or event and being randomly chosen - perhaps the least required effort for a contest possible.
Typically, door prizes will be given away by a business to the millionth or other impressive number customer to walk through the door, with the appropriate fanfare and publicity. Conventions and other ticketed events such as spectator sports usually give away numerous small door prizes by randomly selecting numbers from the tickets sold and then announcing the winners during a slow stretch of the game or when the con organizers want a crowd.
Alternatively, when a bicycle rider goes head over handlebars due to some attentive driver suddenly opening their car door without looking first. Ouch.