One I forgot from the USA - a pair of boilers inside a cheese factory. These two were beasts, the size of them is quite hard to tell from the photo as the room was so cramped.
A rusting external boiler on top of a hill in North Wales. It was part of the old weir system for the Dolgarrog Hydroelectric Power Station. The boiler is situated near the penstocks (water gate valves) where water was piped down to the turbine house of the power station.