The Open Source Development Environment for .NET

#develop (short for SharpDevelop) 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 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.
We are going to add more content (and new releases) over the coming months, so sign up for our announcement list! Also, we want to invite you to participate in the forums, discussing SharpDevelop - its features, feature requests and of course bugs.

Features:

Forms designer for C#, VB.NET and Boo
Code completion for C#, VB.NET and Boo (including Ctrl+Space support)
Integrated NUnit support plus code coverage (NCover)
Integrated debugger
Code Analysis with FxCop
Refactoring support
Multi-framework support (.NET 1.1 and 2.0, Mono, Compact Framework)
XML Editing (source and tree view) with XPath search
Parser-based code converter (C# to VB.NET / Boo and more)
Compile C#, VB.NET & Boo code in the IDE out-of-the-box
Code AutoInsert (Alt+Ins)
Xml documentation preview
Setup projects supported with Windows Installer XML (WiX)
Subversion integration
Open source, LGPL licensed
User interface translated to many languages
Write C#, ASP.NET, ADO.NET, XML, HTML code
Project or File-based development (Project Scout & File Scout)
Rich project options
Syntax highlighting for C#, HTML, ASP, ASP.NET, VBScript, VB.NET, XML
Intelligent braces
Folding
Bookmark your code
Code template support
Component Inspector
Feature-rich Find & Replace dialogs (including incremental search)
Easily extensible with external tools
Easily extensible with Plug-Ins
Re-host SharpDevelop with SDA
... and much more

URL: http://www.icsharpcode.net/OpenSource/SD/