Scott Hansleman has a pretty decent list of software tools that might be useful for developer and regular users in their daily life. My intention with this list is not to copy the existing resource but rather list some .NET-specific tools that developers might find helpful. Some of them require additional downloads, some of them don't - at the end of the day, you are the one who decides whether you need them at all or not. So here goes.

• .NET Reflector - one of the awesome tools that will serve long after it was installed. In my opinion this is the best way to look under the hood of .NET Framework and maybe even find some undocumented goodies. Did I mention that this is also a great way to learn how things work?
• ReSharper - a tool that will significantly increase the coding speed since it offers a large set of actions for code generation, analysis and refactoring.
• LINQPad - a must have tool if you work a lot with databases and OData services and you want to leverage the power of LINQ. SQL-wise, it is faster in some aspects compared to SQL Management Studio.
• Visual Studio Productivity Power Tools - an extension for Visual Studio that makes it easier to navigate through solutions. Tab differentiation with colors (depending on what project it belongs to) is awesome.
• NuGet - now you don't need to manually search for a specific library and add a reference to it. NuGet will automatically download and include the reference to it.
• PowerCommands for Visual Studio 2010 - once again an extension for Visual Studio that adds additional commands to the Solution Explorer that let's you access the project resources much easier.
• Silverlight Toolkit - a set of additional components for Silverlight and WP7 (Silverlight-based) applications.
• MailSystem.NET - if you are looking for a mail library for your .NET project that is able to handle POP, SMTP and IMAP then this is the right choice for you.
• Sandcastle - in my opinion, one of the best ways to generate documentation for your .NET projects.
• PDFSharp - a managed library to work with PDF files, specifically - PDF creation.
• Ninject - dependency injector, one of the best I know (although I still prefer MEF in most of the cases).
• XAMLPad - this tool is installed by default when you install Visual Studio so there won't be any additional actions required to get it. It is great for XAML testing (in the context of Silverlight and WPF).
• NUnit - a well-established and mainteined unit testing framework for .NET.
• xUnit - another unit testing framework for .NET.
• MEFContrib - a set of extensions for MEF (Managed Extensibility Framework)not officially included in the default release.
• Windows Phone Connect Tool - specifically targetting Windows Phone 7 developers, this tool (bundled in the October SDK update) will let you debug media capabilities on the device without Zune Software running.
• JSON.NET - since by default .NET Framework doesn't offer many JSON tools, this library is a gem for those who work with JSON-based responses (especially when working with web APIs).
• NHibernate - a third-party ORM tool for .NET
• SSH.NET - a great library to experiment with SSH commands.
• Snippet Compiler - a minimalistic tool to compile .NET snippets.