Is telnet still relevant? It turns out that the answer is yes. Recently a client commissioned me to develop a telnet client for Android. He's developing a wireless barcode scanner which connects to the server using telnet. To test my app, I needed to enable telnet server on my mac. Here's how I did it:

$ sudo launchctl load -w /System/Library/LaunchDaemons/telnet.plist

And to stop the server:

$ sudo launchctl unload -w /System/Library/LaunchDaemons/telnet.plist