In the recent year I tried using various .Net tools (C#) supplementary to what Visual Studio offers, and the tools listed below really has great functionality. If you know of more useful tools please post a comment.
ReSharper provide you with various code productivity enhancements, it is analyzing code while typing finding and highlighting errors and warnings. Furthermore ReSharper offers code suggestions, analysis of possible flaws in code design, and suggestions to improve them when typing. Key features are Code Analysis for C#, Code Templates, XAML & XML Support, Code Formatting, Code Formatting, Coding Assistance, and Code Refactoring.
Reflector is the class browser, explorer, analyzer and documentation viewer for .NET. Reflector allows to easily view, navigate, search, decompile and analyze .NET assemblies in C#, Visual Basic and IL. Furthermore various add-ins are available to boost the potential and functionality of Reflector.
A dynamic mock object framework for the .Net platform. It’s purpose is to ease testing by allowing the developer to create mock implementations of custom objects and verify the interactions using unit testing. Alternatively use the non-free Typemock.
TestDriven.NET makes it easy to run unit tests with a single click, anywhere in your Visual Studio solutions. It supports all versions of Microsoft Visual Studio and it integrates with the best .NET development tools.
NCover is an essential tool to measure code coverage while creating automated tests, and it helps to test intelligently by revealing which tests haven’t been written yet.
Once you have used NCover to produce an xml result of your coverage, you can open it in this tool to browse the source code with the coverage information highlighted.
Highlights the results of your XPath query in real time against any XML document. Intellisense XPath expression builder. Code samples are provided for mainstream languages (C#, java etc).
Sisulizer is a visual software localization tool, which is able to work directly with your C++ Builder, Delphi, C#, Visual C++, Visual Basic, Turbo languages, Java, Windows binary files, HTML Help (.chm), XLIFF, or with .NET.
dotTrace provides an extremely fast way to profile the performance of .NET applications (frameworks 1.x, 2.0 or 3.x).