!!command

(with NO COLON in front) will replace the current line with the output of the shell command “command”.

Additionally, you can pass text to your command, have it filter it, and replace the text with the filtered output. For example, you might want to send a paragraph to the fmt command:

!}fmt

This will use fmt to format the paragraph (note that using gqap is probably a better solution for this particular example, but it’s just an example).