X-Face

Mozdev.org: Mnenhy - Amongst several enhancements to SeaMonkey Mail/News and Mozilla Thunderbird, brings the possibility of displaying X-Face headers.
http://mnenhy.mozdev.org/

Mythic-beasts.com: X-Face - Includes a brief explanation of the X-Face format and an online Jpeg to X-Face converter along with its source code.
http://www.mythic-beasts.com/tools-toys/xface/

MessageFaces - Extension that adds X-Face and Face image compatibility to Mozilla Thunderbird. Also hosts a FAQ and a quick guide for Face image creation.
http://tecwizards.de/mozilla/messagefaces/

Wikipedia: X-Face - Brief description of this header, its history and a list of compatible software.
http://en.wikipedia.org/wiki/X-Face

Dairiki.org: Online X-Face Converter - Converts various image formats to the X-Face format suitable for inclusion in e-mail or Usenet article headers.
http://www.dairiki.org/xface/

Ashton, James - Personal page of the creator of this format, includes some of his remarks on these images.
http://users.rsise.anu.edu.au/~jaa/

The Face Header - A proposed improvement to the X-Face header. The Face header is a PNG image whereas X-Faces are one-bit images.
http://quimby.gnus.org/circus/face/

A Bunch of X-Faces - A collection of X-Face images and their corresponding text.
http://www.xs4all.nl/~ace/X-Faces/