Examples are numerous: combustion, the thermite reaction, combining strong acids and bases, polymerizations. As an example in everyday life, hand warmers make use of the oxidation of iron to achieve an exothermic reaction:
:4Fe&thinsp; + 3O&thinsp; → 2FeO&thinsp;                         ΔH⚬ = - 1648 kJ/mol
A particularly important class of exothermic reactions is combustion of a hydrocarbon fuel, e.g. the burning of natural gas: 
:CH&thinsp; + 2O&thinsp; → CO&thinsp; + 2HO&thinsp;     ΔH⚬ = - 890 kJ/mol
These sample reactions are strongly exothermic. 
Uncontrolled exothermic reactions, those leading to fires and explosions, are wasteful because it is difficult to capture the released energy.  Nature effects combustion reactions under highly controlled conditions, avoiding fires and explosions, in aerobic respiration so as to capture the released energy, e.g. for the formation of ATP.