C# Tools

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.

DotGNU Project

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

#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.

csUnit

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++.

devAdvantage : C# Source Code Analyzer for Visual Studio.NET

A 3rd party Microsoft Visual Studio.NET Add-in that provides C# static source code analysis to automate code reviews and detects errors, bugs and issues. The Community Edition is a free, fully featured, unlimited use version of devAdvantage that does not contain the threading knowledge pack.

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.