Redis
A very fast in-memory database used almost everywhere as a cache, and increasingly as a memory store for AI applications.
Redis keeps its data in memory rather than on disk, which makes it extremely fast and slightly forgetful by design. In practice it sits between an application and its real database, holding the things that get asked for constantly so the slower system underneath does not have to answer every time. It also handles queues, counters, session data and similar odd jobs, which is why it turns up in so many stacks.
It has become relevant to AI work for a second reason. Applications that search documents by meaning rather than by keyword need somewhere to store the numerical representations of that text, and Redis added vector search for exactly this. It was created by Salvatore Sanfilippo in 2009 and is now developed by a company of the same name.