European NUTS boundaries as GeoJSON at 1:60m
Geo Boundaries for NUTS administrative levels 1, 2 and 3 edition 2013:
core/geo-nuts-administrative-boundaries
If you don't know what NUTS (Nomenclature of Territorial Units for Statistics) are, see the related Wikipedia article.
Country Polygons as GeoJSON
Geodata data package providing geojson polygons for all the world’s countries. Perfect for use in apps and visualizations:
Natural Earth Admin1 Polygons as GeoJSON
Geodata data package providing geojson polygons for the largest administrative subdivisions in every countries:
Natural Earth Admin1 Polygons as GeoJSON US
Geodata data package providing geojson polygons for the states in the USA: