DataDesigner: Standards based .Net document server. With peer-to-peer content exchange abilities. [Public Domain]
DotNetNuke: Framework for creating and maintaining .Net content management web applications.
Lutz Roeder's .NET Reflector: Reflector is an object/assembly/webservice browser for .NET, XML documentation viewer, type browser, custom attribute viewer, multi-source languages, user SDK.
mojoPotal: mojoPortal is an open source content management system and web site framework portal for .NET.
Mono project: Provides the necessary software to develop and run .NET client and server applications on different platforms.
MonoUML - open source .Net Case tool: The MonoUML Project aims to develop a CASE tool for GNU/Linux and UNIX environments, using Mono as development and running framework.
Piccolo Reserach Project: A toolkit that supports the development of 2D structured graphics programs allowing for Zoomable User Interfaces (ZUIs).
Skelta - Workflow: A embeddable .NET based workflow framework
Spring.NET - Application Framework: An application framework that provides capabilities like dependency injection and Aspect Oriented Programming (AOP) for building enterprise .NET applications.
VNC-Client for .NET: A VNC-Client written in C# using the .NET Framework. VNC is a remote display system, which allows the user to access the desktop of a remote machine. [Open source, GPL]