Either inetd or one of the more modern "improved versions" of the Internet Super Server
This standard *nix utility starts network services (daemons) "on demand" - if/when they are not in use, they don't run.
Ref:
http://www.netbsd.org/docs/guide/en/chap-inetd.html
A use case:
You want an httpd daemon (service in en_M$) to run for changing settings remotely, but you don't want to take up the RAM or CPU cycles of having it idling in the background.
This is a good case of using inetd - the httpd daemon doesn't run
until you try to connect to it - then inetd kicks in and fires up a copy for your connection to use.
Can be used for any "on demand" network service.