What are some obscure but useful Vim commands

If my latest post on the topic did not tip you off, I am a Vim fan. So before some of you start stoning me, let me present you a list of “obscure Vim commands.” What I mean by that is: a collection of commands that you might have not encountered before, but that might be useful to you. As a second disclaimer, I do not know which commands you might know and which one you find useful. So this list really is a collection of relatively less known Vim commands.