I've just been evaluating a few.
Crownwoods DotNet Magic.
This has been around for a long time, and was initially one of the best docking managers out there, but its a bit long in the tooth now, we had quite a few crashes, it doesn't allow floating windows to be docked vertically and horizontally, and the tooltips on the tabs have a habit of hanging around.
Telerik
Visually great, but there a quite a few bugs in it, and its got a pretty big foot print (8MB). The event model is a bit difficult to work with as well. I integrated this into our application, only to discard the changes.
Actipro
I recon this is the best of the 3, it supports all the functionality you would expect, and I didn't come across any bugs. It doesn't look quite as good as the Telerik stuff, but at least it works properly, with a sensible object model and event structure.