@simon_brooke @cscottmills @iona @orbifx it's common in many European cities. I've seen it in Belgium, Czech Republic, and Switzerland, and heard of more.
Generally it's a few Euros, which *must* be paid direct to the hotel at the time of settling the bill. You can't pay it in advance, nor as part of any deal with a booking service such as Expedia etc. I believe that's to ensure the money *actually* goes to the local town...