Erlang
Functional language for fault-tolerant distributed systems — Joe Armstrong et al. at Ericsson, 1986, to run telephone switches with nine-nines uptime.
Everything is actors (lightweight processes), all communication is message passing, data is immutable. The BEAM VM schedules millions of processes per node with preemptive scheduling and hot code reloading without downtime. OTP (Open Telecom Platform) is the standard library: supervisors, GenServers, applications.
Powers WhatsApp, RabbitMQ, CouchDB and telco core networks. Joe Armstrong's "let it crash" philosophy shaped modern distributed-systems thinking. Syntax-unfriendly — many hop in via Elixir instead.