Categories of friends and/or tags

Let us create categories of friends. 

Instead of having all friends in one big list, allow us to create sublists (categories) of friends. For example let us put online-gaming friends in one list, real-life friends in another list and friends-of-friends in yet another list.

Another way to create the same functionality would be to allow us to create and attach different tags to friends and let us choose which tags to filter/display in our friends-list.