SocialGmail is a nice little extension for Gmail that displays the profile image of the sender next to their name, helping you to quickly recognize the person.
The image shown by the extension is taken from your contacts – if you have saved an image, or else from the senders Google or Gravatar profile.
The extension does not provide any customization option and merely displays available profile image of the sender.
SocialGmail is available for both Google Chrome, and Mozilla Firefox as a Greasemonkey script.
Download SocialGmail: Google Chrome | Greasemonkey