Ever want ot have the same article in more than one category? Since Joomla 2.5, tags were introduced..
Go to Components>Tags to create your tags. They can be nested under Parent tags.
Just assign the same tag(s) to those articles you want to group together.
Then there are modules and menus that will enable you to present them by tags.
Popular Tags Module (demonstated on sidebar) displays tags used on the site in a list or a cloud layout. Tags can be ordered by title or by the number of tagged items and limited to a specific time period.
Menu items include a List of all tags, or Tagged Items (as demonstated on sidebar).