A few years ago a friend told me about Streak, a Gmail extension that allows you to track whether your email has been opened. For me it was a game changer, simply because it allowed me to have some concept of whether or not a message had made it to the person I intended or had gotten stuck in a spam folder in cyberspace somewhere.
It can be hard to start an email, but it feels even worse when you're going insane after sending the same canned response to a question for the ninth time this week. If you'd like a little help replying to the messages in your ever-growing inbox, EasyEmail uses machine learning (along with your email data) to auto-generate potential replies you're too lazy to type yourself.
I love the idea for me, but hate the idea that people who send me emails could be doing the same (hypocritical, I know).
PixelBlock is a Gmail extension that figures out when someone is trying to track you, notify you the tracking is happening and blocks it.
If for nothing else, it's interesting to see what people and companies have opted to track what they send.