This app was mentioned in
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I was going to link the exact same article. :)
My two cents: most of the time they will give you free tap water for when you are just passing by. If you are really dining somewhere, it's custom to order a bottle of water or a glass and pay for it. But I can't see them refusing you a glass either, so feel free to try it out! Ask politely and don't get mad when they refuse. (And fill in the app if possible so we know where to go if we want some as well!)
EDIT: The app is a Dutch one, and it is user based. If you have any questions, feel free to ask!