Mr. Heater Portable Buddy Heater... I use this thing like crazy! Just takes a normal propane canister, and your tent is warm in minutes. You can hook it out to a propane cylinder too, and run the hose outside. Can be used in your tent, your boat, garage, etc... Make sure to get a bag with it, so it stays clean in-between uses.
Candle baking dish heater. The reflective foil shield pushes heat towards you. The rocks absorb heat and give it off after you blow out the candles. Never leave candles burning when you are about to go to bed, or tired enough to fall sleep. Make sure there is a source of fresh air to avoid carbon monoxide poisoning.