Other than the three main projects, Castle consists of a whole range of smaller, but very useful libraries that you can take advantage of in your application.
Castle DynamicProxy - a lightweight, lightning fast framework for generating proxies on the fly, used extensively by multiple projects within Castle (Windsor, MonoRail) and outside of it (NHibernate, Rhino Mocks, AutoMapper and many others).
Castle DictionaryAdapter - on the fly generates strongly typed wrappers around untyped dictionaries, or chunks of XML (like config file), optionally adding support for change notification, cancelation, error notification and other features.
Castle Scheduler - a lightweight job scheduling service.