DockbarX is a Gnome panel applet that has grouping of applications and live window previews. It is themable and featurefull. DockbarX has improved the way Windows7 implemented grouping and thumbnail previews in its superbar, allowing us to customize almost everything. The latest release looks like page previews in google search with dark color and horizontal alignment. It now provides close buttons in the preview and appropriate thumbnail sizes.
You can install DockbarX in Ubuntu using PPA. Just type the following commands in terminal,