TimescaleDB - TimescaleDB is a time-series SQL database providing fast analytics, scalability, with automated data management on a proven storage engine.
OpenTSDB - OpenTSDB is a distributed, scalable Time Series Database (TSDB) written on top of HBase.
MongoDB - MongoDB (from "humongous") is a scalable, high-performance, open source NoSQL database.
Beringei - High performance, in-memory storage engine for time series data (by Facebook)
Prometheus - An open-source systems monitoring and alerting toolkit.
VoltDB - In-memory relational DBMS capable of supporting millions of database operations per second