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.
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.