The Flickr Shapefile Public Dataset 2.0.
A few years ago Flickr used their geotagged photos to compute shapefiles for each neighbourhood and city they had in their GeoPlanet database. They were nice enough to publish those shapefiles in huge GeoJSON files and put a very nice CC0 licence on it.
I picked those huge GeoJSON files apart using geojsplit and sorted them by their WoEID.
And because I figured someone else might find that useful, too, I decided to upload the whole thing to Github. Enjoy.