Here are three cool plugins you can for transforming default WordPress avatars to something else so that you can get something new when any avatarless reader comments on your blog post. Using following plugins you can add comic avatars like those popular in Reddit or you can display robots and monster avatars or you can only display name of Gravatar-less commenters.
Rage Avatars
Simply install and activate it and it automatically starts displaying Rage Comic Avatars in comments of Gravatar-less commenters.
RoboHash Avatar
The plugin displays Robots or Monsters or both Robot and Monster avatars in Gravatar-less comments. Instll and activate it then visit your admin area Dashboard -> Settings -> Discussions page, scroll down to Avatars section and select RoboHash Avatar under default avatar settings.
Display Name If No Gravatar
It replaces the get_avatar function in pluggable.php to pass an alternate image to gravatar.com that generates the user’s display_name with additional text ‘has no avatar’ as a avatar image if they do not have a Gravatar.
If you also know any good similar plugin then feel to share it here in your comments.