Castle Project is an umbrella for a handful of open source projects for .net. We aspire to simplify the development of enterprise and/or web applications.
While we're celebrating 20 years of @dotnet I think we should recall two very important OSS projects in the .net space: @NHibernate and @castleproject. Great work of many talented people like @ayende , @fabiomaulo , @hammett, and many others