Firefox/Chrome: Google Contacts needs some work, while Google+ Circles in Gmail was supposed to give you quick access to the people you interact with most, but isn't terribly useful. That's where ToutApp comes in -- its new Gmail plugin creates a "Relationships" view where you can see the people you email often, a "Groups" view that automatically creates mini project teams after you feed it a couple of emails, and it can even track the emails you send and let you know when the recipient opens them, no return receipt required.
ToutApp's relationships feature works a bit like previously mentioned Rapportive in that it integrates cleanly and adds a one-click view to the people you email most often, what you discuss with them, and even links to their social network profiles. The new "Groups" feature lets you feed it a couple of emails that you want to keep track of, and it automatically creates a group consisting of the people associated with that chain of messages and the topic you're discussing. This way, you can keep track of the conversation, even if people enter or leave the discussion or the subject line changes.
One of ToutApp's biggest features is that can track your emails and notify you when the recipient has opened them. Just click "Tout It" before sending, and you'll get a notification that includes when they opened it, the system (desktop or mobile) they use and more. ToutApp's primary audience are businesses types (so don't be put off by the impression its site may give you), but the Gmail plugin is free, and you get a limited number of trackable emails each day for free. Yes, you have to grant ToutApp access to your Google Account for the service to work, but you can revoke it at any time. If you're interested, give it a whirl at the link below, and let us know what you thought in the comments.