word: fridges that keep things hot (and not cold)

You are probably referring to a food warmer:

Heat lamps, hot boxes, steam tables, soup kettles and display warmers are just some types of food warmers that currently exist. Heat lamps, steam tables and soup kettles often leave food exposed in some way while they keep it warm. Hot boxes and display warmers fully enclose foods.

Source:http://www.ehow.com/facts_5825763_food-warmer_.html

Since "fridge" is short for refrigerator, it's doubtful you can use "hot fridge" because that would kinda mean "hot cold". An enclosed container that heats or keeps food warm is an oven or a warming oven - as shown below:

It depends on whether you just want to keep warm food that is ready for eating, or whether a warm environment is required as part of the food preparation.

For most of us at home, the oven on a low setting suffices for doing this.

For those involved in the food industry, there are of course more specialised equipment. Josh61 has mentioned a food warmer. If you need to have a warm environment for yeasted dough to rise, you can also get a proofing cabinet.