Maybe you already know it;
there are several docking managers and since April 2009 (3.0) you can include VL Docking for free even in commercial applications, because its LGPL!
The good thing about VL Docking is (and of MyDoggy by the way …) that it is well integrated into the Spring RC project.
More information about VL Docking
All Java docking managers
- Raven Docking Apache 2; 0.2 MB
- MyDoggy LGPL; only jar’s: 0.5 MB
- VL Docking LGPL; 0,4 MB
- NetBeans CDDL/GPL; 4.6 MB (platform.zip)
- Eclipse CPL or EPL ? only swt (?)
- InfoNode GPL or Commercial
- Sanaware GPL or Commercial full zip 0.3MB
- Docking Frames LGPL; 0.7 MB
- Jide commercial; <3MB
Inactive projects
- SwingDocking seems to me fully functional and fast; Apache license 2
- XUI will be further developed here?; MPL; 1.6 MB (XUI-jdk15.zip)
- JDocking CDDL; 1.3 MB (v0.8.zip) the docking part of netbeans
- FlexDock MIT; only jar’s: 0.5 MB
- JRichClient GPL; derivation of flexdock
Please let me know if you know other libraries which support ‘window docking’.
Karussell 
I use DockingFrames, and until recently had regular mails with the maintainer. It’s very much an active project. Looking at the repository, you can see commits in the last 2 weeks, and the guy release fairly often.
Andrew, I updated this accordingly. Sorry, but on the first look at the wikis and files … this looked abandoned…
Hi,
I just released another Java docking manager. 🙂 I wrote this a year ago – at the time, Google searches were only turning up docking managers with restrictive licenses or hard to use code bases. If I only waited a month or two, I might have discovered myDoggy.
Anyhow, it’s called Raven Docking, has an Apache 2 license, and is part of a larger project. Not sure if you want to add it to your list or not:
http://raven.java.net/ravenDocking/index.html
Thanks a lot! added here as well as on SO http://stackoverflow.com/a/1607431/194609