Proton is described as 'tool for use with the Steam client which allows games which are exclusive to Windows to run on Linux kernel and macOS operating systems'. There are more than 10 alternatives to Proton for a variety of platforms, including Linux, Mac, Wine, Windows and BSD. The best alternative is Wine, which is both free and Open Source. Other great apps like Proton are Lutris (Free, Open Source), PlayOnLinux (Free, Open Source), ReactOS (Free, Open Source) and Parallels Desktop (Freemium).
Wine is a free, open source application for running Windows software on operating systems that are normally not supported. With Wine, you can install and run these applications just like you would in Windows.
ReactOS™ is an Open Source effort to develop a quality operating system that is compatible with applications and drivers written for the Microsoft® Windows™ NT family of operating systems (NT4, 2000, XP, 2003, Vista, Seven).
Parallels Desktop gives Mac users the ability to run Windows, Linux or any other operating system and their critical applications at the same time as macOS. Parallels Desktop is optimized for Intel and Apple M1 chip.
Darling — macOS translation layer for Linux Fast Darling runs macOS software directly without using a hardware emulator. Free Like Linux, Darling is free and open-source software. It is developed openly on GitHub and distributed under the GNU GPL license version 3.