Well yesterday I talked about C-o (open-line) that opens up additional
lines beneath the line you're currently on, and how little I used it.
A related key that I use even less, but could see more opportunity for,
is C-x C-o (delete-blank-lines), which collapses repeated lines of
whitespace into a single line.
The first thing I find myself wondering, though, is "Why C-x?" It
seems to me that most other commands like this—variations on a shorter
set of keystrokes—use the C-u prefix to say, "do the opposite-ish".
That aside, I'm going to try and keep this keystroke in mind, because it seems like something I will find a use for, if only I can remember it.