RocksDB
An embedded key-value store from Meta, forked from Google's LevelDB in 2012 and optimized for SSDs and multi-threaded servers. Not a database you connect to, but a library you link in.
The storage model is an LSM tree: writes go to an in-memory memtable and a log, get flushed to sorted files on disk, and are then merged in the background by compaction. That gives excellent write performance at the cost of read amplification and periodic I/O storms, which in turn are governed by a notorious number of tunable parameters. Its significance lies in everything built on top of it: Kafka Streams, CockroachDB, TiKV, YugabyteDB, MyRocks in MySQL and Ceph all use RocksDB as their storage layer. That makes it one of the most widely deployed database engines in the world without ever being visible.