JQ™ Softwares: SharpDevelop

Sharp develop is a free IDE for C#, VB.NET and Boo Projects on Microsoft's .NET Platforms, it is completely open-source and its capabilities could be compared to that of Microsoft's VB.NET Express Editions

Supported Programming Languages

C# (Code Completion, Windows Forms Designer)
VB.NET (Code Completion, Windows Forms Designer)
Boo (Code Completion, Windows Forms Designer)
IronPython (Code Conversion, Windows Forms Designer, partial Code Completion)
IronRuby (Code Conversion, Windows Forms Designer)
F#

Designers

Windows Forms
Windows Presentation Foundation (WPF)
Entity Framework (EF EDM Designer)

Developer Productivity

Refactoring
Parallel build support for multi-core machines
Multi-framework targetting (.NET 4.0, as well as 2.0, 3.0 and 3.5)
Code Autoinsert More information
Starts from a USB memory stick (More information)
Read-only project support (More information)
Full MSBuild support

Quality Tools

Integrated Debugger (including dynamic debugging features)
Code Analysis (FxCop)
Unit Testing (NUnit)
Code Coverage (PartCover)
Profiler (Roadmap)
Integrated Subversion support (TortoiseSVN)
Integrated Git support
StyleCop addin (More information)

And More...

Setup projects (WiX, More information)
SharpDevelop Reports .NET open-source reporting solution (More)
Parser-based code converter (Online Code Converter)
XML "code completion"
Documentation generation (Sandcastle, SHFB)
Reflector addin (More information)
Easy addin extensibility mechanism
SharpDevelop for Applications (SDA) allows you to host SharpDevelop in your applications (More information)
AvalonEdit WPF-based text editor is rehostable to add a programmer's editor to your applications (including code completion!)