Mono and MonoDevelop are alternative C# programming compiler and IDE. They are now owned by microsoft, but at one point was independant.
You will need to download the windows version from this link, it is now called Xamarin Studio:
Below is a screenshot of the files you are looking for:
Make sure you download the .NET Framework 4.5 & GTK# for .NET from the links on the above page before installation.
Other downloads required
When you try to install MonoDevelop / Xamarin Studio you will get error messages concerning MSBuildTools, firstly for the 2013 version and then for the 2015 version. In order to successfully install you can download and install the files below first.
MSBuildTools 2013 - MSBuildTools 2013 Download Page
MSBuildTools 2015 - MSBuildTools 2015 Download Page
You will need to download the Mac version from this link, it is now called Xamarin Studio:
Make sure you also download the GTK+ package listed under Mac.
Get the files
You will need to download the Linux version from this link, it is now called Xamarin Studio:
You will also need to install FlatPak.
You could alternatively manually install it using the command:
sudo apt-get install monodevelop