Apache Kafka
Open-source distributed event streaming platform governed by the Apache Software Foundation. Described as used by "thousands of companies for high-performance data pipelines, streaming analytics, data integration, and mission-critical applications." Core abstractions: topics, partitions, offsets, producers, consumers, consumer groups, brokers. Kafka 4.x runs on KRaft (no ZooKeeper). Includes Kafka Streams (stream processing) and Kafka Connect (integration).