What's your favorite Vim trick?

What’s that Vim trick that blew your mind the first time you learned about it? A feature that has a big (or small) impact on your workflow, or just a command you use a lot. Anything goes! 🤯

I’ll start with my go-to trick: I usually only notice a match should be replaced after searching for it with / (/foo). After learning that substitutions with empty search patterns (%:s//bar/) replace the previously found matches, I’ve never had to re-type a pattern again.