Instance variables in application layout


I have an application layout, and in it there is a sidebar. What i
want to do is put things like tags, newly registered users, recent
comments, etc in the sidebar.

I am not sure how to do this...

What controller does the application layout access instance variables
from? I presume you can use the application controller, but am not
quite sure how...

The controller that is rendering the current action (what other one
could it be?).