A collection of awesome .NET core libraries, tools, frameworks and software (github.com)

There's bottomless open source options for .NET apps, just search nuget, surprisingly often you'll find an open source package that's exactly what you wanted.

I just wish there was a Mono for Android compatible libsodium for .NET.

Also check out https://github.com/quozd/awesome-dotnet which this list was inspired from. This list is .NET Core specific, whereas the one linked above is for .NET in general