Grafana Mimir
Mimir was forked from Cortex to open source some of their work under a more restrictive license (AGPLv3). See https://grafana.com/blog/2022/03/30/announcing-grafana-mimir/ for a feature comparison table.
The most prominent feature is fast high cardinality queries, and a scalable compactor that could support “unlimited” cardinality.
Mimir also did some housekeeping work on the codebase, but this may have been caught up by the Cortex community.
There is also Grafana Enterprise Metrics that supports more features like cross-cluster query federation, but it’s closed source.
Cortex
Cortex nowadays is based on both Prometheus and Thanos code, using them as libraries.
The most important feature Cortex has as mentioned in a 2023 KubeCon talk is improved reliability via tenant isolation with limits, replication and quorum, and shuffle sharding.
VictoriaMetrics
VictoriaMetrics is Licensed under Apache 2.0. It also has an enterprise version that supports more features like downsampling and better automation, observability and integrations. https://docs.victoriametrics.com/enterprise/
Thanos
Thanos is a CNCF Incubating project licensed under Apache 2.0.
Downsampling and compaction is one of the prominent features. Global querying is supported from day 1.
Thanos started only in sidecar mode, running alongside Prometheus instances and allows them to run with relatively low retention by supplementing it with object storage. Later on, the receiver component is added, providing remote write support.
Thanos currently lacks cardinality management tools that Mimir and VictoriaMetrics have, which is tracked in https://github.com/thanos-io/thanos/issues/6007.