If you occasionally need an Isolation Transformer, but you can't justify buying one, then this simple project could be the answer. If you have a couple of old UPSs lying around that no longer work, you can convert one into an isolation transformer using a part from the other.
So, what is an Isolation Transformer? In simple terms, you feed in 230VAC from the mains and you get out 230VAC that isn't connected to the mains. Why would you want to do this? The mains power has three wires, Active, Neutral and Earth. Neutral and Earth are at ground or zero potential, whereas Active is at 230VAC, which can kill you.
This construction project shows how you can make your own Isolation Transformer.