ECSY - Small, simple ECS framework with focus on performance. Not a full game engine, just an ECS system.
Artemis - Java ECS framework, also just an ECS system, not a full engine.
Entt - Header-only C++ ECS framework. Again, just an ECS system.
Flecs - C ECS framework, bit more full featured with components, systems, queries, etc.
Unity - Yes, Unity uses an ECS under the hood. The new Unity DOTS system exposes this more directly.
Bevy - Rust ECS-based game engine.
Amethyst - Rust data-driven game engine using an ECS.

So there are a few options for open source ECS-based engines, as well as just ECS systems to build on top of. Ultimately a "game engine" is a quite broad concept, so it depends how much you're looking for out of the box vs flexibility to customize.
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