ASCII Art

ASCII Art diagrams are created using ASCII characters. WikiPedia has an entry about ASCII Art:

ASCII art is an artistic medium that relies primarily on computers for presentation and consists of pictures pieced together from the 95 printable characters defined by ASCII.
ASCII art is used wherever text can be more readily printed or transmitted than graphics, or in some cases, where the transmission of pictures is not possible. This includes typewriters, teletypes, non-graphic computer terminals, in early computer networking, e-mail, and Usenet news messages.

There are many commercially available programs that can convert images or text that you type, into ASCII Art diagrams.
I really like the one that's available online: http://www.sebastian-r.de/asciiart/

JavE - Java ASCII Versatile Editor is a free program that could be used to create ASCII Art diagrams. You will need Java Runtime to run JavE. You can download JavE from here: http://www.jave.de/