Are there any existing desktop applets or tools which give a nice overview of the content of workspaces similar to that in the new Gnome3?
Similar to this:
If you use Gnome2 you can activate the compiz plugin EXPO and / or SCALE.
Expo displays all workspaces. You can move windows from one to another. Scale displays all windows of all workspaces. I use both.
You have to install "compizconfig-settings-manager" to activate the plugins and set your preferences.
I'm not sure if this is similar enough, but the workspace switcher in Unity Ubuntu 11.04 provides a visual overview of all workspaces, and allows windows to be dragged from one workspace to another.
It can be invoked by clicking the workspace switching icon in the Unity panel or with the shortcut Super-s