SharpDevelop (#develop) is a free IDE for C#, VB.NET and Boo projects on Microsoft's .NET platform. It is open-source, and you can download both sourcecode and executables.
Mono (mono-project) is a software platform designed to allow developers to easily create cross platform applications. Mono is an open source implementation of Microsoft's .NET Framework based on the ECMA standards for C# and the Common Language Runtime.
Visual Studio Express 2013 for Windows Desktop. Lets you take full advantage of Windows with XAML designers, a productive IDE, and a variety of programming languages including C#, Visual Basic, and C++. Choose between Windows Presentation Foundation (WPF), Windows Forms, and Win32, to target the Windows desktop with the right technology for your application and your skills.
Try Visual Studio Express 2015 for Windows
QuickSharp A minimalist IDE for Microsoft .NET QuickSharp is a streamlined integrated development environment for Microsoft's .NET platform. It provides a simple, uncluttered development approach that lets you get programming in an instant. QuickSharp may be downloaded free of charge from SourceForge.
live long and coding...