For Bash, M-# (alt-shift-3) puts a comment token '#' at the start of the current line and begins a new empty line. Handy if you are 150 characters deep in a command and realise you need to do something else for a moment (i.e. momentarily in the correct sense). You can store your 150 char command and come back to it, it is in the history and can be retrieved with a couple of Up-arrows.