A variety of different geo shapes as GeoJSON
Original source from http://www.arnulf.us/PLZ/
Original source from http://www.naturalearthdata.com/
Mapshaper http://mapshaper.org/
GDAL ogr2ogr http://www.gdal.org/ogr2ogr.html
ogr2ogr -f "GeoJSON" file.geojson file.shp
Convert Shapefile to GeoJSON