OuluBox
The University of Oulu's BBS, whose chat feature IRC was written to replace.
OuluBox was a BBS run by the Department of Information Processing Science at the University of Oulu in Finland. It had the usual parts — message boards, files, news — plus a real-time chat called MUT, MultiUser Talk. MUT worked, but only for people logged into that one machine.
In the summer of 1988 a student named Jarkko Oikarinen was given charge of the system and started writing a replacement. His model was Relay on BITNET, and the goal was a chat that could span several servers. The result was IRC, which promptly outgrew its mothership. OuluBox is one of the network's neater examples of a feature built for a local service outliving that service by decades.