A mainstay for any .NET programmer is the Visual Studio IDE.  This tool has numerous bells and whistles which allow you to build, debug, test and deploy .NET applications. 

Of course, VS does cost money, and many new .NET programmers feel they must spend a goodly amount of cash to build .NET software.  Thankfully, nothing is further from the truth. There are countless free tools (many of which are open source) which can be used during the development process. 

Here is a hit list of some of the very useful tools you might like to check out first hand.

SharpDevelop: This is an open source IDE, which allows you to build .NET applications. You can download the source, or a friendly setup program.

CSharp Express: This is a free .NET IDE from Microsoft (yes, you heard that correctly, "free...from Microsoft").

Reflector: A very feature rich object browser, decompiler, source code generation tool. Must have!

Kaxaml: If you are interested in WPF development, you will want this free tool, which allows you to enter in any valid XAML markup, and see the results real time.  Very helpful way to learn the grammar of XAML, without the overhead of a Visual Studio WPF project.

LinqPad: If you are interested in LINQ, this graphical tool allows you to enter LINQ queries and view the results.

This is just a small sample of tools out there to download and play with.