There are many useful, practically essential, open source solutions available to .NET developers. This is a list of the ones that have been the most useful in my experience. Agree? Disagree? Leave a comment with your own suggestions.

1. NHibernate - NHibernate is a mature, open source object-relational mapper for the .NET framework. It's actively developed, fully featured and used in thousands of successful projects. http://nhforge.org/Default.aspx
2. NUnit - NUnit is a unit-testing framework for all .Net languages. Initially ported from  JUnit, the current production release, version 2.4, is the fifth major release of this xUnit based unit testing tool for Microsoft .NET. It is written entirely in C# and has been completely redesigned to take advantage of many .NET language features, for example custom attributes and other reflection related capabilities. NUnit brings xUnit to all .NET languages. http://www.nunit.org/index.php
3. jQuery - jQuery is a fast and concise JavaScript Library that simplifies HTML document traversing, event handling, animating, and Ajax interactions for rapid web development. jQuery is designed to change the way that you write JavaScript. http://jquery.com/
4. Rhino.Mocks - 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. http://ayende.com/projects/rhino-mocks.aspx
5. MVC Contrib - This is the contrib project for the ASP.NET MVC framework. This project adds additional functionality on top of the MVC Framework. These enhancements can increase your productivity using the MVC Framework. It is written in C#. Founded by Eric Hexter and Jeffrey Palermo. http://www.codeplex.com/MVCContrib
6. CruiseControl.NET - CruiseControl.NET is an Automated Continuous Integration server, implemented using the Microsoft .NET Framework. http://confluence.public.thoughtworks.org/display/CCNET/Welcome+to+CruiseControl.NET
7. S#arp Architecture - Pronounced "Sharp Architecture," this is a solid architectural foundation for rapidly building maintainable web applications leveraging the ASP.NET MVC framework with NHibernate. The primary advantage to be sought in using any architectural framework is to decrease the code one has to write while increasing the quality of the end product. A framework should enable developers to spend little time on infrastructure details while allowing them to focus their attentions on the domain and user experience. http://code.google.com/p/sharp-architecture/
8. Spark View Engine - Spark is a view engine for Asp.Net Mvc and Castle Project MonoRail frameworks. The idea is to allow the html to dominate the flow and the code to fit seamlessly. http://sparkviewengine.com/
9. TortoiseSVN -  A Subversion client, implemented as a windows shell extension. TortoiseSVN is a really easy to use Revision control / version control / source control software for Windows. Since it's not an integration for a specific IDE you can use it with whatever development tools you like. TortoiseSVN is free to use. You don't need to get a loan or pay a full years salary to use it.  http://tortoisesvn.tigris.org/
10. Castle Windsor - Castle Project offers two Inversion of Control Containers. The MicroKernel and the Windsor Container. Castle Windsor aggregates the MicroKernel  and exposes a powerful configuration support. It is suitable for common enterprise application needs. It is able to register facilities and components based on the configuration and adds support for interceptors. http://www.castleproject.org/container/index.html