What is Castle Project?
Castle is an open source project for .NET that aspires to simplify the development of enterprise and web applications. Offering a set of tools (working together or independently) and integration with other open source projects, Castle helps you get more done with less code and in less time.
Castle ActiveRecord is the enterprise data mapping pattern implemented using NHibernate.
Castle MonoRail is a MVC web framework that lets you rapidly build testable and maintainable applications.
Castle Windsor is best of breed Inversion of Control (or as some like to call it - Dependency Injection) container. As part of the project, there are also a handful of facilities, which extend Windsor and/or integrate it with other tools.
Castle DynamicProxy is very fast and lightweight runtime proxy generation framework. It is used by other projects under the Castle umbrella, as well as numerous other projects, like NHibernate, Moq, Rhino Mocks and many others.
Castle Services and Tools help you bootstrap your infrastructure easily.
Outside of Castle Project there's a bunch of so called contrib projects that are not part of Castle Project but add or alter its functionality.
Castle is free
All projects of Castle Project are available for free, no strings attached.
Better yet - Castle Project is Open Source, under the terms of Apache License 2.0, which means you are free to use it in any closed source, commercial product.
Castle's source code is available. You can look at it, or better yet - fork it, improve, extend, do whatever you like with it. If you want to contribute your changes back to the project read Castle coding standards and how to submit a fix to any Castle Project.
It's community driven
Castle has a vibrant community of users that contribute back to the project by: