vvvv

A visual live-programming environment for .NET [Free for OSS]

Consulo

A cross-platform IDE with C# & Java support, fork of IntelliJ IDEA Community Edition

RoslynPad

A simple C# editor based on Roslyn and AvalonEdit.

Rider

A cross-platform C# IDE based on the IntelliJ platform and ReSharper

Ionide

An Atom Editor and Visual Studio Code package suite for cross platform F# development.

Visual Studio Code

Excellent open source editor from Microsoft, based on Electron.

Waf DotNetPad

A simple and fast code editor that makes fun program with C# or Visual Basic.

Visual Studio Community

A full-featured open source IDE

MonoDevelop

MonoDevelop is a cross-platform IDE mostly aimed at Mono/.NET developers

SharpDevelop

A free IDE for .NET programming languages

AvalonStudio

An extensible, cross platform IDE written in C# for Embedded C/C++, .NET Core, Avalonia and Typescript