Is it safe to put hot food in the fridge?
This is a myth left over from the days of iceboxes. Go to any official food safety resource online, like the USDA or FDA, and you will find that they are all in agreement: it's perfectly safe to put hot food in your refrigerator. In fact, unless you are using some more direct cooling method, like putting your food in an ice bath, waiting to refrigerate your food is often a health hazard.

For example, see 


FDA — Despite what some people believe, putting hot food in the refrigerator doesn't harm the appliance.
FDA — A lot of people think it will harm their refrigerator to put hot food inside—it's not true. It won't harm your refrigerator and it will keep your food—and you—safe*. (Page not found)
USDA — Hot food can be placed directly in the refrigerator or it can be rapidly chilled in an ice or cold water bath before refrigerating 
Washington State Department of Health — Fact: Hot food can be placed in the refrigerator. 
Partnership for Food Safety Education — Fact: Hot foods can be placed directly in the refrigerator. (Page not found)
Alaska Food Safety and Sanitation Program — Myth #10 "I can't put hot food into the refrigerator. The food will spoil if I do." —
The food will spoil if it is not quickly cooled! The leading cause of foodborne illness in the United States is improper cooling, such as leaving cooked foods at room temperature … (Page not found)
Florida Department of Health — Hot food can be placed directly in the refrigerator if you divide large quantities into small, shallow containers. (Page not found)


These are just a few links I found in the first couple pages of a search; there are obviously many more. IMPORTANT NOTE: Particular precautions should be taken for large quantities of food; see below for details. But there are three common objections: (1) it will "overwork" the fridge, (2) it will heat up other food in the fridge to unsafe temperatures and cause it to spoil, and (3) somehow, the uneven cooling of the hot food in the container will cause it to spoil. Let's take these each in turn.

"It will overwork the fridge" — False in most cases

Lots of people worry about putting a quart or two of hot food in their fridge, but they wouldn't think anything of filling their fridges with multiple gallons of room temperature drinks to chill them, even if it might require the same amount of energy to cool.  Anything that is above the temperature of the fridge will require energy to cool.

Now if you plan on putting a very large amount of hot food into a small fridge all at the same time, it might strain the fridge. If you do it once in a while? Probably not an issue in a modern fridge, as long as it's not excessive. If you plan to do that on a regular basis, you should consider using ice baths or other cooling methods to pre-chill the food before putting it in the fridge.

It will cost more energy to cool hot foods in the fridge. If you are looking for the most energy-efficient method, it is probably to use a cold tap water bath, replacing the water a few times to bring down the temperature as fast as possible to room temperature. Then refrigerate.  Chilling with ice water is safer and faster, but it's not the most energy efficient, if that's your concern.

Also, in most cases it probably makes sense to wait until the food actually gets into the "Danger Zone" (below 140°F or so) before refrigerating; there's little safety benefit in putting a boiling pot directly into the fridge. Unless you're monitoring the temperature of your cooling food, though, it still is safest to move it to the fridge as soon as you can.

"It will heat up other food in the fridge to unsafe temperatures and cause it to spoil" — False in almost all cases.

This is the real myth that comes from the icebox theory. In an old icebox, you just had a block of ice, and no air circulation. If you put something hot in there, the ice would melt, the food wouldn't even get cool, and everything else would warm up and spoil.

That just doesn't happen in a modern fridge, as can be found out on reputable food safety websites, such as mentioned above. I've actually tested it myself. I once put a gallon of very hot soup in the fridge just to see what happened. I checked the fridge every 30 minutes or so by measuring surface temperatures of other foods with an infrared thermometer.  My fridge is generally around 35–36°F.  There was one item which was almost touching the metal pot that maybe reached 44°F or 45°F for a short time—above ideal refrigeration temperatures, but hardly enough to cause a lot of spoilage. All other items in the fridge—even many on the same shelf—stayed within a few degrees of the refrigerator temperature, 40°F at the most.

And this wasn't in some fancy fridge: it was actually at least 10 years old and a rather cheap model. The air circulation is enough to keep most other foods cool, unless you're actually touching them to the hot container. The radiative heat coming off the hot container will raise stuff around it by a couple degrees for a couple hours, but in a fridge kept at a proper low temperature, this should not be an issue at all.

If you have a very old fridge that doesn't work properly, or some weird new fridge that is super energy-efficient and doesn't cool properly, you might have an issue. But for anyone with a normal modern fridge, this is not an issue.

EDIT: Another question was posed in response to my comment here, and I documented there a more detailed experiment I carried out by placing a gallon of boiling water directly into my home refrigerator (admittedly a newer model than in the last experiment).  My results, in summary, were that even food items placed within a couple inches on the same shelf of the pot of boiling water were only heated by 3–4° for a few hours.  Other items in the fridge (on other shelves, etc.) barely changed temperature, moving maybe a degree or two at most.  These temperature fluctuations might be a minor concern around highly perishable foods like raw meats, but it is probably common sense to keep raw meat away from hot food and warm food in general.  Other items in a well-functioning modern refrigerator are unlikely to be negatively affected. If you don't believe me, here's another documented similar experiment with a large hot cheesecake put directly into the fridge from the oven.

"The uneven cooling of the hot food in the container will cause it to spoil" — Certainly false, compared to room temperature cooling.

In fact, leaving your container out on the counter at room temperature has a much, much, much higher probability of causing faster spoilage.  With some dishes, it is actually quite irresponsible.  There's a common belief that cooked food is "sterile," and by leaving it out with the lid on, it won't get contaminated.  But there are a lot of microbes that produce spores which can even survive boiling or near-boiling temperatures.  Many of them are kept at bay by competing with other microbes during the initial cooking phase, but after cooking, all that's usually left are nasty spores.  

Rice, for example, is commonly infected with Bacillus cereus, which forms spores that aren't destroyed during normal rice cooking.  Leave a rice dish out on the counter for too long, and you'll start growing a lot of that stuff, which can produce persistent toxins that won't be destroyed during reheating.  This sort of thing is actually the cause of a lot of food poisoning from eating things like leftover Chinese food: if the rice was cooked earlier and not held above 140°F, and then was allowed to sit in a take-out container for many hours, it could grow a lot of toxins.  It's often more likely to get food poisoning from the leftover rice than from a lot of other dishes that might seem more "dangerous."

Rice and Bacillus cereus are just one example.  You really want to get food down to refrigerator temperatures as quickly as possible after cooking.  Don't just leave it on the counter.

If you have a large quantity of hot food, break it down into small containers, and then refrigerate. Not all stacked together though; let air circulate around them.  Or put your pot into an ice bath or cold water bath.  Stirring periodically in an ice bath will make it cool even faster.

It's true that in a large pot of chili or something, the temperature of the center of the food will go down much more slowly and potentially could allow bacterial growth compared to outer layers.  That's the reason that large pots of food should be broken down into small containers.  But note that leaving a pot of chili out on the counter will make this bacterial growth even worse than if it is refrigerated immediately.

Even if you don't do any of these things and leave all the hot food in one large pot, it's still a safer strategy to put it directly into the fridge rather than leave it out on the counter at room temperature.

Important Edit: For large quantities of hot food, I am NOT advocating the practice of placing a large container in the refrigerator.  It may take WAY too long for the large pot to cool completely, and the food in the large container may be unsafe to eat.  Either break it down into smaller containers or use an ice bath.  However, it's still unlikely to harm the food in the fridge or the fridge itself unless it's in direct contact with the hot container.  And it's still safer than leaving the food on the counter to cool.