I always loved how people can do this. I have tried a few times and just can't manage the simplest of designs!
Feel free to post ones you think are amazing!
This one has to be one of my favorites I have ever seen:

Here are some 3D foam cats. Not sure if they use something extra for this or not but it looks cool!
My favourite ones are the ones with colour. I don't see it done very many places but a number of cafes do it in Korea. Kangbin Lee ins particular does amazing art work in every cup.
Look at how stunning this is:



