Is there any food out there that I can eat as much as I want and won’t gain weight?
Why would you ask this of Japan’s bicameral legislature? 
I think you can eat as much of just about anything you want – as long as you expend at least as much energy as you consume.
Funny, funny guy 
What would be a super-low calorie food that I could eat a heaping bowlful of? Celery? Cucumbers? Carrots?
When I was growing up there was a myth that it took more calories to process celery than was in it, so you could eat as much as you wanted and you would still lose weight. I believe this myth has been busted, and even if it hasn’t, how much celery can one person eat a day before they die of boredom?
I snack on carrots, celery and other raw vegetables… because they are better for you than just about anything else you can think of.
Kelp is very, very low in calories. You can basically eat it without concern of weight gain. Never tried it myself, so can’t tell you if it tastes any good.
I was serious about the second part.
Celery has about 19 kilocalories per cup. Cucumbers have about 16 kilocalories per cup. Carrots have about 52 kilocalories per cup. If the average number of kilocalories needed by an American male is 2,000, then you can have 125 cups of cucumber, 105 cups of celery, or 32 cups of carrots per day. Of course you need to get your recommended daily allowance of vitamins, minerals, protein, etc. So a single-item diet is not recommended.
The thing is, people are different. An active person might burn 5,000 kilocalories per day, while a sedentary one will burn less than average. So no matter what you eat, you need to burn at least as many calories as you take in if you don’t want to gain weight.
Plain, raw celery, lettuce, broccoli come to mind. Cabbage–although I don’t know if you’d eat it raw–as well. These seem to be foods that in order to over-consume on calories would take eating such a great volume, it might not be physically possible to do.
I can eat a whole bag of potato chips easily, for what, 1500 calories?
If I were to eat 1500 calories of absolutely plain cooked potatoes, I’m sure my belly would burst.
Spinach. 40 calories per cooked cup. I don’t know if it’s even physically possible to consume 1500 calories worth of spinach. That’s like…12 quarts of cooked spinach.
A cup of raw spinach has 7 calories.
Well, the simple answer is no.
The longer answer is that every food has a different calorific content, and the fewer calories the more you can eat. None have zero calories.
Your body needs energy (calories) to operate, of course. Without those you’ll die.
The average person needs about 2000 calories per day. More if you’re active, less if you’re sedentary. A pound of fat is equal to 3500 calories, so for every 3500 calories above or under that 2000 per day you eat, you’ll gain or lose a pound of fat.
That’s an incredibly simplistic description, but it’s a good starter.
If I’m hungry late at night I’ll snack on a cuke with soy sauce, some sauteed mushrooms or cabbage.
I wonder if the OP is asking about sustaining oneself on a single food alone, which would be inadvisable and impossible, or about having a food that he or she could eat until they were absolutely stuffed that wouldn’t cause weight gain. If it’s the latter, I still think any low-calorie, high-fibre, high water content vegetables like the ones cited above would suffice.
If I could take that advice to heart, I would be full of salads and a sylph.
How many Calories in a sylph? Would you still have room for a whole one after the salads?
Salt has zero calories! I recommend you install one of these in your back yard.
Failing that, raw green vegetables are your best bet. Make sure you get adequate protein and carbohydrates as well, though.
Ice.
It actually does have (very slightly) negative calories, since your body has to expend energy to bring it to body temperature. There is a limit to how much water you can take in for other reasons, though (too much can kill you, but I think it would be hard to get to that level by eating ice).
There exist a number of zero-calories flavorings and sweeteners, so you could probably do something like snow cones. And as previously mentioned, you could salt it.
This could be a very, very unpleasant diet.
Would it then still be classified as “food”?
There are any number of totally non-nutritious bulk items that could be eaten without gaining weight. Cardboard might be an example.
Most things sold as “food” are, by design, nutritious in some way, although some of them might have minimal calories per unit of weight, either because they are largely water or, like celery, not very digestible for humans.
As an aside, I don’t personally believe in the bulk approach to satiety for those considering dieting. Over time, it seems to me this would increase the size of the stomach and make it increasingly difficult to be sated. My 2c, which in the dieting business is not even worth a penny.
I don’t know what the technical definition of “food” is, but I’d be willing to bet that water does not fall into it. 
Yes, the second one. Since I’m dieting and eat like a horse, what is something that I cat eat a lot of at night while watching TV and not worry about weight gain.
I realize that all foods have calories and that if I ate bushels and bushels of anything I would gain weight. I am talking about a snack that within all practical limits of eating it won’t cause weight gain.
I’ve found that microwave popcorn (the stuff without butter) goes a long way for a snack. But raw vegetables, as others have said, are you best choice.
I’d like to add Hot peppers to the list. I promise you’ll eat as much as you like (dare to eat) and not gain weight.