This is the way it works, a new deal is emailed to you each day, the deal is not "on" until a certain number of people buy it, so you may want to share the deal with friends if it's a deal you want in on. Once enough people decide to buy it, the deal is on, and you will be billed. Deals are limited in time and number so you must act quickly. Once you've purchased the deal, you generally need to print a coupon to take to a store or restaurant. The discount may expire in a year or even six months, so read the fine print carefully before you buy.
All the of the following sites offer deals in San Francisco, and are adding cities all the time so check to see if your city is listed as well. Many of the sites also offer you a referral fee if you get friends to sign up and they make a purchase.
Groupon (think "group" and "coupon") is the largest of these sites. Recent deals in San Francisco include $25 for $50 worth of food at Luna Park, $15 for $30 worth of food at Kasa Indian Eatery, and $10 for $20 worth of food and drinks at Goat Hill Pizza. There are also deals for spas and entertainment. Groupon offers deals in over 50 major cities in the US.
TownHog has recently offered deals such as $20 for $40 worth of food at Ramblas and $10 for $20 worth of hot beverages, like their scrumptious hot chocolate, at Bittersweet Cafe. You'll also earn $10 for each friend who signs up and purchases a deal. TownHog offers "hot deals" in 42 cities.
My Joffer offers another similar program. Recent deals include $17 for $33 worth of cupcakes at That Takes the Cake and $15 for $30 worth of pizza at Bruno's Pizzeria. They also have a lot of spa specials. MyJoffer offers deals in about 17 cities and some deals are nationwide.
Living Social offers sends out daily deals, and a recent offer was 50% off Indian cuisine at Rotee. You can also use the site to keep track of collections of things such as a virtual bookshelf where you collect, rate and review books that you own. An added feature is an iPhone app if you prefer to get your deals sent directly to your phone. Living Social deals are available in 8 cities but the other features are available everywhere.
Fresh Guide doesn't have as many restaurant deals available as some of the other sites, but the ones they do have are very good, such as a $25 for $50 worth of Italian cuisine and drinks at Sociale. Fresh Guide is available in 8 cities so far.
One more!
This morning a friend told me about BloomSpot. BloomSpot offers hotel discounts in addition to spa and restaurant deals. Recent restaurant deals include $20 for $40 worth of food at Colibri, $10 for $25 worth of food and drinks at Roots and $15 for a $30 prix fixe dinner at Heaven's Dog. BloomSpot is only available in San Francisco, Los Angeles and New York, but they will be adding more cities.