onenetd is a single-server inetd for POSIX-style operating systems, similar to Dan Bernstein's tcpserver or courier-tcpd from Courier-IMAP.
onenetd supports both IPv4 and IPv6.
onenetd allows you to limit the number of concurrent connections, optionally refusing further connections with a configurable message — this can be used, for example, to limit connections to busy FTP servers. (onenetd ran the UK Mirror Service's FTP servers between 2002 and 2012, serving millions of connections in that time.)
The included manual page describes all of onenetd's options. I'm aware the name is hard to spell and pronounce; feel free to suggest a better one!
There is a mailing list for onenetd users and developers, upon which new versions of onenetd are announced and discussed.