In most campaigns, I'll have it occassionally as a curiosity. Usually I'll use some form of real-world tech that is more 'advanced' than was the case in our world at around the late middle ages/early Rennaisance: dwarven steam engines, something like a hang-glider, the wate closet, etc. Stuff that would be very plausible for them to make normally, but either we never thought of it or it was regarded as a momentary curiosilty and then dumped for 200 years or more.
Most fantasy tech is some kind of ancient device, but the occassional wizard will decide that, dammit, I am going to have shaved ice on shipboard and I'm a wizard so I'm going to do something about that; he binds an air elemental into a wooden crate and creates the deep freeze.