If you use Gmail you will love Rapportive. Rapportive is a Gmail plug-in released for Firefox and Chrome which replaces the Gmail advertisements with information about the person you are corresponding with. It also shows what social media platforms people are in so you can easily grow your Twitter, Facebook, and LinkedIn networks.
Check out the screenshot below which I got off of their website.
Rapportive doesn’t require your Gmail credentials, it uses a secure Google Federated Login or Open ID. It doesn’t have access to your password, but it does have access to your emails in order to gather information. Rightfully, this may turn some people off, but remember that any extension that you add to your browser also has access to anything you do on the Internet.