Akka
Actor model implementation for JVM. Created by Jonas Bonér in 2009. Inspired by Erlang. Builds Akka Streams, Akka HTTP, Akka Cluster, Akka Persistence.
Lightbend (formerly Typesafe) is the commercial maintainer. Heavy users: LinkedIn, PayPal, Walmart. Akka 2.7 (2022) changed license from Apache 2.0 → BSL (Business Source License) — controversial, spawned a fork (Apache Pekko). User base has shrunk since. Pekko is a drop-in replacement. Competitors: Vert.x (event loop), Project Reactor, Project Loom (virtual threads make the actor model less necessary).