GMAT (NASA GSFC): General Mission Analysis Tool: https://sourceforge.net/projects/gmat/
Orekit: https://www.orekit.org/overview.html/ (This tool has an extensive well-written document.)
poliastro - Astrodynamics in Python: https://github.com/poliastro/poliastro
CelesTrak: https://celestrak.org/
Space-Track.org: https://www.space-track.org/
LEOLabs.space: Low Earth Orbit Visualization: https://platform.leolabs.space/visualization
PreviSat: Satellite tracking software: https://sourceforge.net/projects/previsat/
Gpredict: Real time satellite tracking and orbit prediction: https://sourceforge.net/projects/gpredict/
TBA.