Steam Chat Logger
http://www.swbfgamers.com/index.php?action=downloads;sa=view;down=1329 (http://www.swbfgamers.com/index.php?action=downloads;sa=view;down=1329)
WARNING: The current versions of steam do not work with this logger. I will update if there is any work around, but at the moment, it seems otherwise.
All credit to the creator of this tool. (I am not.)