Visual Studio: 
From Microsoft is a must for serious .NET developement. There are free version as well (the express line, as in: C# Express, VB Express, ...)
Another free althernative (other than the express suite) is #develop.

NUnit:
Is a unit test framework for .NET. I suggest every solution should have a test project using NUnit, where we could store all automated test as create them.
It also comes with a nice GUI which gives a quick visual feedback of what's wrong and what's not.

Reflector:
A tool to decompile .NET assembly, very informative and usefull.

FxCop:
A tool which check various things relative to the "code quality" of an assembly. Although this is a debatable concept, it occasionaly comes with very usefull information such as: performance tips, proper IDisposable and Serializable implementation advice, dead method detection and so on...

GhostDoc:
A tool which help write some boiler plate XML code comment. No magic, as far as I can tell it write a default template or take the comment from the interface or superclass whenever appropriate.

Sandcastle:
A tool to build class documentation from the XML documentation. It's a command line which is, apparently, difficult to use. IT why it's a good idea to also install Sandcastle Help File Builder which use sandcastle in the background, but comes up as a nice GUI application.

TortoiseSVN:
A Windows Eplorer integrated SVN tool.
To use the "svn+ssh" protocol, right click on a folder (in the file explorer) => TortoiseSVN => Settings => Network
In the SSH client command line add the parameter:
 -l <your SVN user account> -i "<you Putty RSA private key file path>"

The putty RSA private key is generated with puttygen and an RSA private key.