EventStoreDB
Database purpose-built for Event Sourcing + CQRS — immutable, append-only event streams. Greg Young created it 2012.
Differs from regular DBs: events as primary storage, projections derived. Strong in DDD-leaning enterprise applications. Open-source core + commercial Enterprise Edition. Used by Banco do Brasil, Sky, Vodafone, NHS England. Competitors: Marten (PostgreSQL + .NET), Axon Server, custom Kafka-based event stores.