We've used Restaurant.com for years. The way it works is that you buy a coupon that gives you a dollar amount off the meal, and the price you pay for the coupon is way less than the face value. Typically, you can get a $25 certificate for $10 or a $10 certificate for $4. Towards the end of the month, they'll often run 70% or 80% off specials (all you have to do is enter the promo code that they distribute to their e-mail members) so you can get them much cheaper.
We've run into very few problems over the years. One time we had a restaurant refuse to honor the certificate, saying that they had opted out of the program but that Restaurant.com had not removed them from the site, but they later sent us a gift card as an apology.
They recently made a change that I think is awesome. In the past, you had one year to use a certificate after purchasing it, otherwise it expired. Now, they have removed that restriction, so as long as the restaurant is open and still participating, you can use the certificates. From what I understand, any previously expired certificates are now valid as well. There are two certificates that we never got around to using that I had written off (I think they were $10 certificates that we'd paid $0.80 each for) but I'm now going to go back and attempt to print them, as I know both restaurants are still open and both participate.
So, if your eyes were bigger than your stomach and you never got around to using some of your certificates, go back and check, because you may be in for a pleasant surprise!
Thanks to my dad for passing this tidbit along!