The challenge is open to any software, but here's a list of popular open-source tools to consider. No programming skills are required. Or if you are a programmer, no design skills are required.

• QGIS
  • A Free and Open Source Geographic Information System. A desktop software that allows you to read/write multiple data formats and output (mainly static) maps.
• Kepler
  • Open source geospatial analysis tool for large-scale data sets and for interactive maps.
• Aerialod
  • Not really a GIS tool, but can be used to create some stunning 3D visualizations
• Blender
  • Free and open source 3D creation suite. Check out the BlenderGIS extension.
• R packages for geospatial
  • sf, ggplot, tmap and rayshader to mention few examples.
• Python packages for geospatial
• geopandas, xarray-spatial, matplotlib, geoplot, hvplot and plotly to mention a few.
• Mapshaper
  • Useful for geospatial data processing in the browser such as file format conversion, map projection, feature simplification, filtering, clipping, merging etc. Can also be run locally from the command line.
• Vega-Lite
  • Declarative visualization specification including cartographic output. Specifications can be written directly in JSON or via program language interfaces such as altair (Python) and elm-vegalite (Elm). See, for example, this tutorial on using OpenStreetMap with elm-vegalite
• Vega
  • More flexible but lower-level declarative visualization specification including cartographic output. Specifications can be written directly in JSON or via program language interfaces such elm-vega.
• Litvis
  • Literate Visualization notebook environment optimised for specifying visualizations and documenting the design process. While not specific to cartography, can be a useful environment for exploring cartographic design.
• Observable
  • Reactive notebook environment for generation of visualization and cartographic output embedded in a textual narrative. Focus is on using d3 for specifying visual output.