FIGlet is a neat little command line tool (CLI) that allows a user to create ASCII-type letters in a variety of fonts. I use this in documentation, for example, to mark a new block; for XML/XHTML a tiny shell script does the work for me:

#!/bin/sh
( echo "<!--"
figlet $* | sed -e 's/^/  ** /'
 echo "  -->"
 )

The program is available for a large number of platforms including Windows (with an unescesary GUI), MS-DOS, and of course for Linux/Unix as source code. If you want to experiment a bit without downloading FIGlet, there is an online FIGlet Server as well.