Kotatsu is a special table with an electric heater mounted underneath. A heavy duvet is placed between the table frame and the tabletop. Kotatsu are so commonly used to get the Japanese (who don't have central heat) through winters that Rement released a kotatsu toy.
The warmth and coziness of kotatsu are awesome. Leaving a kotatsu behind, however, can feel like torture. Marukai markets sell square kotatsu for $228.