The Archigos data on political leaders
  Non-State Actor Data
  Expanded Population data
  Expanded Trade and GDP data
  The Cshapes dataset provides historical maps of state boundaries and capitals since 1945. The CShapes R package allows researchers to perform various useful operations on the GIS maps, including calculating minimum distances between states
  The Gleditsch and Ward Minimum Distance Data (see also Distance between Capital Cities Data).
  The Polity data archive, courtesy of Ted R. Gurr
  The List of Independent States outlined in Gleditsch, Kristian S. & Michael D. Ward. 1999. "Interstate System Membership: A Revised List of the Independent States since 1816." International Interactions 25: 393-413.
  A revised list of Wars Between and Within Independent States conforming to revised Gleditsch and Ward list of states