Last minute restaurant suggestions for NYC

The Clam, Ivan Ramen (not the Slurp Shop). Plus Ivan is close to Russ and Daughters so you can pick up some appetizing.

Be nice to the host and try to grab a seat at a bar, if you can't score a table. You can also check out the newly launched Resy.

Restaurants often have cancelations, so arm yourself with the phone number of your target restaurants and try calling around noon time or in the afternoon, and you might get a table of your dreams. Opentable will also have more openings a day or two before, as the cancelations come in.

If you can be flexible, you'd be surprised. But ideally, you need to show up in person (vs Open Table or other apps). Thurs, Fri and Sat nights are difficult. You may need to do some zooming around town. Be sure you're familiar with the cab situation. (Check out the articles in the little side panel.)
http://nymag.com/news/features/taxi-hailing-apps-2014-7

Babbo, Mario Batalis flagship. At 5:30, you can get in and grab one of a few tables in the bar area, or a couple of spots for dinner at the bar. There are no reservations for these seats, as the turn over during the night you can grab one.
If there are just 2 of you, you can get seated at the bar at Union Square Cafe, Gotham Bar & Grill, ABC Kitchen, Lafayette, Lure Fish Bar, and many other great restaurants. I live in Manhattan, and my wife and I have eaten at all of these places and many more, without reservations for an unplanned night out.

If you're willing to eat really early or late (like 6 or 10 pm), your last minute chances are better. But I'd call several places directly - ask to be on their waiting list if there's a cancellation. Person to person, odds are best, I've found - and obviously, the more flexible you are about time, the better your chances. Are you only interested in Manhattan?

Northern Spy is one of my favorites. They take reservations but always save some tables for walk-ins.

Maialino is a company favorite!

Budget, party size and location? Or ambience I guess

Sometimes its easier to make last minute reservations when some folks have canceled the rezzies they've double booked... (bad people!!!) Or you could try again today for next weekend. I always prefer to make a reservation even if it's only a couple of hours ahead. Are you trying on Open Table or by calling the restaurants?