Mono is described as 'free and open source project led by Xamarin (formerly by Novell and originally by Ximian) to create an Ecma standard compliant .NET Framework-compatible set of tools including, among others, a C# compiler and a Common Language Runtime' and is an app in the Development category. There are six alternatives to Mono for a variety of platforms, including Windows, Linux, Mac, Online / Web-based and BSD. The best alternative is Qt, which is both free and Open Source. Other great apps like Mono are .NET Framework (Free, Open Source), .NET Core (Free, Open Source), Dolphin Smalltalk (Free, Open Source) and CrossBrowdy (Free, Open Source).
Qt is a cross-platform application framework that is widely used for developing application software that can be run on various software and hardware platforms with little or no change in the underlying codebase, while having the power and speed of native applications.
The .NET Framework is a software framework developed by Microsoft that runs primarily on Microsoft Windows. It includes a large library and provides language interoperability (each language can use code written in other languages) across several programming languages.
.NET Core is a cross-platform free and open-source managed software framework similar to .NET Framework. It consists of CoreCLR, a complete cross-platform runtime implementation of CLR, the virtual machine that manages the execution of .NET programs.
Dolphin is a complete development IDE for Smalltalk targeted specifically at Microsoft Windows. It is capable of creating standalone GUI and console executables that will run on any version of Windows from XP up to the current Windows 10.
.NET Micro Framework for Linux (NETMF Linux) is sample porting code including complete OS facility. We provide the code for Eddy, Embedded ARM architecture Linux board. The source codes can be ported to any other architectures and platforms, if the cross-toolchain is available.