AvalonDock is the best open source WPF docking library available. Currently, the project has been taken over by Xceed from the original developer. I have a repo on GitHub of the last commit and have started contributing to AvalonDock by adding VS2012 dark and light themes.
I also actively monitor the issues of AvalonDock Codeplex to see if I can fix any for the community before it is taken over by Xceed team.