PostgreSQL
Open-source relational database known for correctness, robustness, and extensibility. Traces back to Berkeley POSTGRES (Michael Stonebraker, 1986).
Officially "PostgreSQL" — pronounced "post-gress-Q-L", but most people just say "postgres". v1.0 in 1996. Avalanche of success after the 2010s: the default choice for modern startups + serverless. Extensions turn it into nearly anything: PostGIS (geospatial), TimescaleDB (time-series), pgvector (vector search for AI), Citus (distributed). The hate exists: no industry standard for "Postgres as SaaS" — every cloud provider has their own flavor.