Give your Custom Controls a Design-Time Support Part 1 & 2
I have a friend called Amr he started recently blogging about building Custome Controls and give these controls design time suppot. He is digging deep on to provide design time for custome controls. To have a closer view, visit his blog and read his design time support posts (Part 1 and Part 2).
Also you can read his aticle about buildin Progress Disk control it is realy awesome