Aerospike
NoSQL database built to answer in under a millisecond at millions of operations per second, by keeping the index in RAM and the data on SSD.
Started in 2009 as Citrusleaf and renamed in 2012. The niche was brutally specific: real-time ad bidding, where a request must be answered in a few milliseconds or not at all. The trick is a hybrid memory model — the primary index in DRAM, the records themselves on flash, with a storage engine that bypasses the filesystem and writes straight to the device.
The result is extreme throughput per node, which keeps clusters small even at huge volumes. Open source since 2014. It competes with Redis and Cassandra, but wins precisely when the latency budget is a hard ceiling rather than a wish.