Groupon is a free program. If you sign up, you will get notices about local deals where a group discount rate has been negotiated. You can opt to buy in at the low price or ignore the offer. It's a cool concept. It is customized to your location.
Groupon is the biggest and the original, but another one has sprung up....
BuyWithMe. It's also free, and there are deals for the following cities: Boston, NYC, San Diego, Washington DC, Phoenix, Philadelphia, Los Angeles, Austin, Dallas, Houston, Las Vegas, Chicago -- choose your city after you enter your zip. If I were near any of these cities, I'd sign up for both Groupon and BuyWithMe