If you ever thought previously mentioned Xobni looked cool, but you prefer Gmail to Outlook, free Gmail plugin Etacts adds many of the same features. You get social information, conversation history, and advanced sending preferences right in your Gmail sidebars.
The Etacts plugin automatically adds detailed contact information to the sidebar of messages, as shown above, similar to previously mentioned Rapportive, but Etacts takes it one step further. Not only do you get links to any social networks that contact is a part of, and some of the information contained therein (such as their occupation and location), but you also get a detailed summary of your mailing history with them, complete with nice little graphs and charts. All this information is also available in compose mode as well, so you know exactly who you're sending it to.
The plugin also adds a few new convenient features to Gmail related to follow-up messages. If you have a particular issue that you need to keep an eye on, you can opt to send the message and have Gmail remind you if you haven't heard back within a specific period of time. It also adds a category in your left sidebar for all messages awaiting reply, so you can view them all at once if you want to.
As with most other plugins, you do need to allow it to access your Gmail inbox. However, Etacts uses Gmail's OAuth, so you log in with one click and it never sees or store your password. The service does store your email subjects, from, to, and time headers (obviously, since it needs to display these on the sidebar), but never the bodies—the team has made it very clear to us that they take privacy seriously, and assure us that the information they do store is never shared with anyone else, either.
Etacts is available as a free extension for both Firefox and Chrome, but if you use a different browser, you can still get all of Etacts' features with the Etacts bookmarklet, also available on their site—just click it every time you open up Gmail and want Etacts' features. The plugin works for both Gmail and Google Apps accounts, although it won't work in both at once (you can only be logged into Etacts with one account at a time).