This plugin provides a large randomly assembled avatar for each user based on their email address. When a user comments on the site, the plugin generates a unique avatar based on his email. There are an impressive number of avatars that can be generated, and it increases at each update of the plugin!
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FL3R FeelBox: https://wordpress.org/plugins/fl3r-feelbox/
FL3R User Agent Comments: https://wordpress.org/plugins/fl3r-user-agent-comments/
FL3R Randavatar: https://wordpress.org/plugins/fl3r-randavatar/