I already posted a list of interesting visualizations for the email. But those were all (academic) prototypes and not used by the general public. However, several Gmail extensions are already available that provide some visualization and insight of the email. Do you know any other? Please let me know if I missed something interesting.
"Rapportive shows you everything about your contacts right inside your inbox. You can immediately see what people look like, where they’re
based, and what they do. You can establish rapport by mentioning shared
interests. You can grow your network by connecting on LinkedIn, Twitter,
Facebook and more. And you can record thoughts for later by leaving
Similar to the above Raportive: "Get names, pictures, social networks, and work information with every email. Search and find contacts from everywhere, even if they’re not Gmail contacts."
Inbox Influence: https://inbox.influenceexplorer.com/
"A new tool that allows you to see the political contributions of the people and
organizations that are mentioned in emails you receive. This easy-to-use
tool can be used for researching influence background on corporate
correspondence, adding context to newspaper headlines or discovering who
is behind political fundraising solicitations."
Etacts (not available anymore, but still interesting): Lifehacker article
Allowed to see social information, conversation history, and advanced sending preferences on the left
Graph you inbox: http://www.graphyourinbox.com/
"a Google Chrome extension that allows you to graph Gmail activity over
time. You can use it to visualize your communication with friends, your
Facebook activity, when you purchased items on Amazon or how often you
use certain words or phrases. We provide the same search functionality
used by Gmail, but instead of a list of messages we show you a graph of
your email trends over time."
MailBrowser (also not developed anymore :() http://mailbrowser.com/