1. Restaurants.com - I use this website constantly. On this website you can search by zip code for $25 certificates to restaurants. I only buy if I have a discount code. Normally you would have to pay $10 for a certificate, but often times with a discount code I can get it for only $2.
2. Groupon.com - I am fairly new to this site, because they only offer good deals in big cities. Often times it is for other stuff, not restaurants. About once a week they offer a good deal on dining so it is worth checking out if you live in an area they offer services in.
3. LivingSocial's Daily Deals - This is the same idea as groupon. It is only available in most cities, including European cities. They offer an array of deals, many of which are restaurants.
4. Entertainment book - They have these in all kinds of cities and there are always a lot of restaurant coupons.