Free C# Tools
Visual Studio Express 2013 for Windows Desktop
The updated version of Studio Express lets you develop using the latest versions of Visual C#, Visual Basic, or Visual C++. Support for Windows 8 desktop, Windows Presentation Foundation (WPF), Windows Forms (WinForms), and Win32.
Microsoft Visual C# 2010 Express
Available along with the other tools in the Microsoft Visual Studio 2010 Express suite. It can be downloaded for free with registration required within 30 days.
The DotGNU project aims to be for webservices and for C# programs what GNU/Linux is rapidly becoming for desktop and server applications. DotGNU Portable.NET, an implementation of the Common Language Infrastructure (CLI), more commonly known as ".NET", includes everything that you need to compile and run C# and C applications that use the base class libraries, XML, and Systems.Windows.Forms. Currently supported CPUs: x86, ppc, arm, parisc, s390, ia64, alpha, mips, sparc. Supported operating systems: GNU/Linux (on PCs, Sparc, iPAQ, Sharp Zaurus, PlayStation 2, Xbox,...), *BSD, Cygwin/Mingw32, Mac OS X, Solaris, AIX
C# Builder Personal Edition
For non-commercial Windows development only. I'm pretty sure you need to register for free and give them a valid email before you can download.
#develop (short for SharpDevelop) is a free IDE for C# and VB.NET projects on Microsoft's .NET platform. It is open-source (GPL), and you can download both sourcecode and executables from this site. In addition, you can find the latest information and changes on #develop, as well as get in touch with the team in the forum.
Inspired by JUnit, csUnit brings the power of unit testing to the .NET framework. csUnit is your key to test-driven development using .NET languages such as C#, Visual Basic .NET, Visual J#, or Managed C++.
Ximian C# compiler
The Mono Project is an open development initiative sponsored by Ximian that is working to develop an open source, Unix version of the Microsoft .NET development platform. The Mono C# compiler is considered feature complete at this point and relatively mature.