site visitor location map

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I'm trying to figure out how to create an interactive map, one that
allows anybody to place a pushpin at their location.  If possible, they
should be able to change that location whenever they like.

I've seen this before on some websites that try to track where their
visitors are based.  Site visitors can click on their location on the
map, and a pushpin will appear there for all to see.

Does anybody know how to create something like this?

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