I am looking for a program which I can use to create ASCII diagrams such these:

It should preferably be a package available in Debian. The wonderful diagram displayed above is taken from the Linux kernel documentation. I cannot believe they were created by hand. There must be some tool to create them.

asciio
I've used asciio for several years. Many of the diagrams on this site I've created using asciio.

example

vncviewer         .-,( ),-.
   __ _         .-(          )-.           gateway           vncserver
[__]|=| ---->(    internet    )-------> __________ ------> ____   __
/::/|_|        '-(          ).-'        [_...__...°]       |    | |==|
                     '-.( ).-'                               |____| | |
                                                             /::::/ |__|
The GUI looks like this.

NOTE: Everything is driven from the right click menu as well as short-cut keys.

DrawIt
Using vim along with the DrawIt plugin you can also create basic diagrams. A good overview of how to install and use it is available here in this article titled: How To Create ASCII Drawings in Vim Editor (Draw Boxes, Lines, Ellipses, Arrows Inside Text File).

asciiflow
There's a website called asciiflow which is probably the easiest way to draw these types of diagrams.

JavE
Another tool, JavE, written in Java that can create ascii diagrams like this as well.

                                 ,'''''''''''''|
                                 | Controller |
                                 |             |
                                 '`'i''''''''''
                                  ,'        `.
                                ,'            `.
                               -                -
                     ,'''''''''''''|      ,''''''''''''`.
                     |    Model    |______|    View     |
                     |             |      |             |
                      `''''''''''''       '`'''''''''''''
The GUI looks like this: