EnTT

Michele Caini released EnTT in 2017. Each component type gets its own sparse set: a dense array of components plus a sparse index keyed by entity id. Adding or removing a component is therefore almost free, unlike archetype-based designs that move the whole entity between tables. The bill arrives when iterating over several components at once, which EnTT softens with "groups" that keep the sets sorted in matching order.

It is a building block, not an engine, and best known as the ECS core of Minecraft Bedrock Edition. It has also become the reference point in the recurring argument over which storage model is actually faster — the answer being, as usual, "it depends".