Gilt is brilliant. So are things like groupon/scoopst. Throw in woot as well. What other areas can this be applied to? The usual factors can include:

a) Email newsletter delivery

b) Scarcity of time

c) Discounted price

d) Possible exclusivity

e) Very very many : 1 (or a few)

f) Possible locational constraints involved

g) An incentive to invite others.