Restaurant.com - use area code 10001 to find restaurants in the Manhattan area. I recommend L'Ybane and AJ Maxwell's. Once you sign up with them they will email you coupon codes to use. I usually only pay $1, $2 or $3 for a $25 gift certificate.
Groupon - Everyday is a new deal. They offer deals on a little bit of everything.
LivingSocial - Choose NYC Midtown and they'll have all kids of deals: food, shows, spas, tours, and more. There is a new one available everyday.
Buy With Me - this is a great website for getting discounted shows, tours, restaurants and more.
Scout Mob - they too also have a variety of deals, but some of theirs are completely FREE. They also text your coupon to your phone.
Zozi - this one has all kinds of stuff including some really nice side trips like snow shoeing, hotels, dancing and more.
EverSave - mostly savings on products which comes in handy if you plan to do some shopping!
Social Steals - deals on pub crawls, tours, spa, dining and more
and I almost forgot to include CityPass! For just $89.00, CityPASS offers a savings of 46% on 6 must-see New York attractions!
Goldstar also offers discounted tickets in NYC.