I’m currently studying up on the Castle Project. It is currently scheduled for release in January of 2007, but the public API code is pretty much complete; it won’t change.
There is a good article on InfoWorld and I’ve found 2 others on CodeProject (1 | 2). The second one from CodeProject talks about Castle on Rails. I believe this is now called MonoRail, mimicing the Ruby on Rails Framework. The article made it look VERY easy to setup a website with Monorail. A co-worker recently mentioned he would try to setup a new site with the tool and see how it works out. I may just do the same thing.
If you are not familiar with Inversion of Control, the first article from CodeProject and a recent article from MSDN are worth the read. They highlight usage of the Windsor Container, the IoC control from the Castle Project.
Good IoC Explanations/Articles:(this will be updated throughout today) Martin Fowler Wikipedia DevX
Open Source Inversion of Control Containers in C#: StructureMap: written by Jeremy Miller, one of my favorite bloggers off CodeBetter.com. Castle: discussed above Spring.NET: another IoC container, based on the Java Spring Framework