This is a joke, but a real service like this could be neat. For example, it listens on all ports and returns a "null" response based on the typical service of that port.

Not-very-well-thought-out-examples:

http - Always returns 404.

https - Always returns 404.

ipp - Returns whatever IPP says for "printer not found" or equivalent.

lpd - Takes data, does nothing with it.

dns - Always returns NXDOMAIN.

ident - Always returns nobody:nogroup

telnet - Accepts any username/password/credentials, returns "This account is not available" and immediately exits.

ssh - Accepts any username/password/credentials, returns "This account is not available" and immediately exits.

smtp - Accepts message, doesn't do anything with it.

pop3/imap - Accepts any login, always has 0 messages.

irc - Accepts any nick, PRIVMSG or other commands, commands don't actually do anything. Returns null list of channels and users when queried.

vnc - ?

rdp - ?

Etc.