Hydrogen Balloon - In addition to its obvious crowd appeal, detonating a hydrogen balloon can be used to demonstrate combination, redox, formation, and exothermic reactions and activation energy.
Limiting Reactant - This demonstration is usually used in a General Chemistry course when the concept of limiting reactant is being introduced.
Emission Spectra - samples of various elemental gases are placed in a high-voltage power supply. This demo is usually performed when the electronic structure of the atom is being discussed.
NO2/N2O4 Equilibrium - Heating or cooling flasks of NO2 and N2O4 shifts the equilibrium between these two species. This demonstration can be used when the concept of the equilibrium concept is being introduced to illustrate the temperature dependence of the equilibrium constant.
Flame Test - Ethanol solutions of metal salts are sprayed into a burner flame producing brilliant fireballs with the characteristic color of each metal. This demo is usually used when discussing atomic structure to illustrate that electron transitions
Frozen Board - barium hydroxide octahydrate and ammonium chloride, freez wet beaker to board. A great example of an endothermic reaction. This demo is usually performed when thermochemistry or thermodynamics are being discussed.
Thermite - spectacular reaction iron oxide and aluminum undergo a single replacement reaction producing great quantities of heat and a stream of molten iron and aluminum oxide. Can be used to demonstrate an exothermic, single replacement, or redox reaction.
Magnesium and Hydrochloric Acid -commonly used when discussing the characteristic reactions of either metals or acids. It's also a great example of a single replacement reaction when discussing types of reactions.
Iron and Oxygen
This is a very versatile demonstration. It is an example of a formation reaction, a redox reaction, and a combustion reaction.