door prize

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A prize awarded by lottery to the holder of a ticket purchased at or before a function.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A gift given to people who attend a meeting, party, event, etc., especially one which is given on a random basis to one or some of the attendees.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun tickets are passed out at the entrance to a dance or party or other social function and a prize is awarded to the holder of the winning ticket