GeoJSON

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:

core/geo-countries

Natural Earth Admin1 Polygons as GeoJSON

Geodata data package providing geojson polygons for the largest administrative subdivisions in every countries:

core/geo-ne-admin1

Natural Earth Admin1 Polygons as GeoJSON US

Geodata data package providing geojson polygons for the states in the USA:

core/geo-admin1-us