Pidgin IM client comes to my mind where there was a feature (not sure if only for Yahoo Messenger or also for other services) that when someone started to type,

Pidgin IM client comes to my mind where there was a feature (not sure if only for Yahoo Messenger or also for other services) that when someone started to type, their chat window appeared for me. People were shocked about how I knew they were going to message me. Would be fun to have something like this on slack.