5

The Task Manager has three options for grouping windows: Do Not Group, Manually and By Program Name. What I think would be really useful and intuitive is grouping the windows that are already grouped by the titlebar tab thing (Middle-click dragging the titlebar of one window into the titlebar of another window joins the windows together and the titlebar behaves like a tab bar). This would make a lot of sense as the Task Manager items are supposed to represent windows.

This might be a bit difficult to implement, though.