Funnily enough there are actually multiple Slack-like chat app projects underway in the Meteor community. I guess building a chat app is the new building a todo list app? Yesterday at Meteor's main meetup Devshop this was presented: http://spacetalkapp.com/ and the speaker mentioned that there's a big community effort underway to keep improving it. Let's see which one wins the race :)
A chat application is pretty much the canonical "Hello, world!" of socket programming. No wonder it is becoming the "Hello, world!" of web sockets too.
Normally, the progression is time-server, echo-server (with timestamps, sometimes), "selective echo server" (don't reply to sender, allow multiple recipients/subscribers) -- aka "chat".
What fun is message passing if you've got no one to pass messages to, or receive messages from? And when you're doing that, you have chat! :-)