Upon news of RocketLab's successful flight of the Electron launch vehicle, it was announced that not only had they deployed the three onboard payloads into the desired orbit, they had done so using a newly developed kick stage powered by an engine called Curie:
The successful launch of Rocket Lab’s Electron rocket Jan. 20 also tested a kick stage designed to circularize the orbits of its satellite payloads. [...] The heart of the kick stage is Curie, an engine developed by Rocket Lab.
From SpaceNews' article: Rocket Lab launch also tested new kick stage.
An image of the Curie kick stage can be seen in this photo supplied by RocketLab:
However, the kicker is that this doesn't appear to be using the typical monopropellant of Hydrazine:
The heart of the kick stage is Curie, an engine developed by Rocket Lab. Beck described the engine, capable of performing multiple burns, as 3D-printed and using an unspecified “green” monopropellant. [Emphasis mine]