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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
VMware ESXi Free (or vSphere Hypervisor) is a great option, however, it has limitations. The main limitation is that storage api is limited, and you can't use Veeam or alternatives to backup VMs. You can use ghettoVCB with free ESXi. https://github.com/lamw/ghettoVCB
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.