The outline view of the standard Eclipse editors is also supported for the Scala IDE ! Hit Alt + Shift + Q, O to bring it up. You will be able to see eclarations for package, class, member, etc.
The outline view is also linked to your editor : clicking on members in the Outline view focuses your editor to the region of text where that element is declared. In a large file, this may be very useful.
A lighter version of the same feature is available if you press Ctrl + O (Command + O on Mac) with the focus on the current editor.