Dogbolt is one of the most impressive free online resources. It allows you to upload an EXE file and see the output of open-source decompilers simultaneously.
(I recommend including this disclaimer to manage user expectations, as "decompiling" is often misunderstood) exe decompiler online free portable
: Another powerhouse for .NET. While it has an installer, the community often provides portable versions (or you can use the dotnet tool version) to browse and decompile assemblies without a full setup. Dogbolt is one of the most impressive free online resources