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Free chess engines:
Stockfish Polyglot - Machines rapid competition and analysis (updated)
SugaR - Machines rapid competition and analysis
CorChess - Correspondence chess or long games (updated)
asmFish - Machines rapid competition
Cfish - Analysis but obsolete (on machines with many threads Stockfish 11 is better)
Brainfish - Machines rapid competition
Raubfisch - Machines rapid competition
These are the best and the strongest 2020 chess programs. They are totally free.
A Graphical User Interface (GUI) is required to use them, such as Fritz (Windows), the free Arena (Windows and Linux) or Droidfish (Android): any of these programs (chess engines) must be installed on said interfaces.
How to install these programs on Fritz: https://www.youtube.com/watch?v=Sohmt3X8ezs&feature=youtu.be.