Gmail plugin enables tracking when and where you open your email.
The ability to use remotely loaded images in HTML emails for tracking has been known for years, but perhaps not widely known.
The On The Media: TLDR podcast just re-surfaced the issue in the above article, where they talk about a free Gmail plugin called Streak, which provides this capability.
It automatically embeds the hidden images in emails you send, then lets you see when and even where the recipient opens them.
Because they appear to use IP address based locations, you can block the “where” part by using Anonymizer Universal.
You can block this tracking completely by turning off the loading of images in your emails. Of course, if you then choose to load images, know that you are also enabling tracking. If you block image loading you will also find that your email become much less attractive and significantly more difficult to read.