bash

1. you select some text

2. you press ctrl+p, type "apply shell command"

3. you type (for example): "nl -ba"

4. highlighted text is echoed into the command you gave, its output is soaked up, and the highlighted text is replaced with the output.

5. et voilá! you have just prefixed all lines with a line number.

other useful commands would be fmt, sed, grep -Eo etc.

I could code this myself in Python but maybe someone will beat me to it.