The one from Codeproject is the AvalonDock – we use it for more then half a year now, but we’re far from release yet so we have the flexibility. Before ending up with AvalonDock we tried Infragistix, ActiPro, SandDock and may be some others.
Even though AvalonDock is not 100% bug free (well what is?) there are no major ones, it is very stable, fast and has all the functionality. It does support multiple docking sites.
Its an open source project and is in active development, so bugs are beeing found and fixed. Good experience so far.