I am working on a project that just begs for a dockable window interface and, fortunately, there is a pretty nice open source solution to that exact problem called “Dock Panel Suite”
It is really easy to get up and running and, in less than 4 hours, we had converted our entire MDI application into a dockable MDI application (ala Visual Studio). It’s really quite nice. A nice bonus is that it is still actively maintained.
The only real drawback is a dearth of documentation. However, the sample app does help get you rolling.