Jetpack is an official WordPress plugin directly from Automattic. It's an interesting plugin in that it doesn't just do *one thing* — it does a whole slew of things that enhance what your WordPress site can do. *Any* WordPress site, that is, and often with extremely little effort. Those easy win features Jesse Friedman calls light switch features, meaning you literally flip a switch in Jetpack's settings and start benefitting. I love that.
There are dozens of features in Jetpack, and I personally make use of most of them and see the benefit in all of them. Allow me to share with you five of my favorites and how they are actively used right here on this site. It's actually a bit hard to pick, so perhaps I'll do this again sometime!