Recommendations on a WPF Docking Library
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.
I've been using the ActiPro library for several months and it's done me well. It does support multiple docking sites. The support is outstanding and you get some other controls (date picker, etc) that are missing from WPF. To me, for $150 it's money well spent. It worked out of the box, no fuss.
We used to use Divelements for WinForm controls but we think Actipro has better support, so we switched for WPF.
Just my two cents.