What's your most-used terminal trick nobody taught you?

[tkn (thom)]

If bash scripting is included, this one:

:||cat<<BASH_MULTILINE_COMMENT_TRICK

To add multiline comments,
without needing a '#' preceding every line.
and without processing 'here document' at all.

BASH_MULTILINE_COMMENT_TRICK

EDIT: Can also be written as:

:||:<<BASH_MULTILINE_COMMENT_TRICK