[REQUEST] Category to sort topics

I would like to categorise my topics so they can be viewed by 'genre' or grouped. An example of what i want :
http://forumz.tomshardware.com/hardware/forum-c2.html
> You see that the 'General motherboard' forum is categorized in 'brand' subforums (or category), but when you open the 'General Motherboard' forum you can see all posts from different brands.

Using the sub-forum mod, I think it could be achieved by making all the 'subforums' topics to still appear in the 'forum'.  Anybody know how to do that?