I did this a few years ago, a simple tool that pinged places all over the world and calculated where you should be.

This only ever gets as close as your ISP, or perhaps a local routing center if your ISP has multiple (mine has only one in Amsterdam, I live on the other side of the country). This means it's less accurate than your typical GeoIP database.

Edit: I'm no longer sure this is the same. The title mentions "passive", and from another comment I understand that the author meant that you can find someone's location if you are the man in the middle, observing traffic to various places around the world. This is definitely something I had not considered yet!