A lot of my job is building WebForms and websites. Yes, we're a MS shop so I'm using ASP.Net and it is a very powerful tool. The extension model which allows developers to create custom server controls is wonderful - and I've even gone and used some concepts normally found only in Rich-Client applications in my web applications.
So, of course I knew that you could extend and build custom controls in Windows Forms in the same way one could do it in Web Forms, but DevX has an article up on it here. All in all, it's not a bad tutorial on doing it.