Dock is an addon that, as the name suggest, allows you to shrink and dock other frames into it. On mouseover frames can be zoomed in letting you take a good look at their contents.
Features: Virtually infinite amount of docks, or just one if you'd like Practically any frame can be docked by just dragging and dropping Highly customizable: colors, animations, orientations and more Tabbing feature that lets docks be collapsed even further Load on demand configuration, no reason to waste that memory
You can view a video of Dock here: http://www.youtube.com/watch?v=nzBz60OtZKo
You can view a video of Dock's tabbing features here: http://youtube.com/watch?v=tZC74v4U2ng
The Dock wiki with FAQs is here: http://www.wowace.com/wiki/Dock
Dock is still considered beta, please treat it as such.