Docking for Linux has been implemented by the project
GnuStep.
The
WindowManager WindowMaker has been built on this framework and you can find a lot of docking applications on
http://www.bensinclair.com/dockapp/
Personnaly I am found of wmfishtime to have a nice clock with little fishes and wmbubbles to monitor my cpu activity and my memory with nice bubbles and a duck swimming.