1) .NET Reflector

Reflector is used to examine and decompile. NET assemblies in IL, C#, and Visual Basic. You can view the class hierarchies of .NET assemblies, without having source-code for them. This is a must have for any .NET developer.

2) NUnit + TestDriven.Net

TestDriven.NET is a tool that is used to run unit tests with this plugin. It is integrated with Visual Studio.

3) LINQPad

LinqPad lets you query databases in a LINQ in real time usingin C# or VB.

4) CruiseControl.NET

CruiseControl.net is an open source tool for continuous software builds for .Net projects.

5) RhinoMocks

Rhino.Mocks is a .Net mocking framework for the .Net platform.Its purpose is to create mock implementations of custom objects which makes testing easier.

6) Fiddler

Fiddler is a web debugging proxy which logs all HTTP(s) traffic between the computer and the internet.

7) NCover

This tool is used for finding out how much code is covered by automatic tests.

8) NDepend

This tool is used for dependency analysis on .NET applications and results are presented in a TreeMap.

9) log4net

log4net is a port of the popular log4j framework to the .NET runtime. It provides granular control over log management and configuration.

10) PostSharp

PostSharp is an AOP framework for the .Net platform.

11) LogParser

Log parser is a powerful tool that provides universal query access to text-based data such as XML and CSV files. It can also be used to query data sources such as the Registry ,Event Log and the file system.

12) StyleCop

StyleCop is an open source static code analysis tool from Microsoft . It checks C# code for compliance to StyleCop's recommended coding styles.

13) Process Explorer

Process Explorer is a helpful utility program that the computer's running processes.

14) NAnt

NAnt is an Ant like build tool for .NET