Potassium permanganate and a drop of glycerin will burst into flames after a few seconds. This can be used to light thermite it gets so hot. A similar reaction will occur with sugar and KMnO4 when it gets wet. Any KMnO4 mixture is very sensetive and shouldn't be mixed beforehand. Ammonium chloride, ammonium nitrate, and zinc dust will burst into flames of they get wet, so if you use that, don't mix the three beforehand. If you only need a lower temp like 210F, a few drops of concentrated H2O2 on iron filings, even turnings will get intensely hot very fast, but not as hot as the KMnO4 since high temps will vaporize the remaining H2O2 and cut the reaction short. It is self limiting. The trick is to let a small amount of rust form on the metal beforehand, which seems to catalyze the reaction. I have not done this last one myself, but have heard about it a lot. This isn't a reaction (well, the battery is I suppose), but short circuiting a 9volt with a piece of nichrome wire will make the wire red hot almost immediately.