Well, I just found Half Off Depot for Atlanta, GA. This is the first site of its type that might be worth the investment: Jolie Kobe, Fritti, Meskerem, Bhojanic (!), and Cafe Intermezzo are just some of the restaurants available.
If I'm planning an outing with friends in advance (do I have any other sort of outing? :) ), it might be worthwhile to decide on the restaurant and buy one of these certificates.
Food for thought, as it were. Have any of you used this sort of discount? If so, what did you think?