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.