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This reddit points to Rocket Lab's news page Rocket Lab successfully circularizes orbit with new Electron kick stage. It is named Curie, presumably after Marie?

According to the Rocket Lab's article:

The kick stage is designed for use on the Electron launch vehicle with a payload capacity of up to 150 kg and will be used to disperse CubeSat constellations faster and more accurately, enabling satellite data to be received and utilized sooner after launch.

Equipped with a precision pointing cold gas reaction control system, the kick stage also has its own avionics, power and communications systems.

It sounds a little bit like a rocket-propelled cubesat dispenser. Is that what it is? What sort of propulsion does it use?

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uhoh
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From the RocketLab site:

(this is a view of the bottom of the kick stage, with a tiny rocket nozzle in the center) Curie kick stage

An apogee kick stage that can execute multiple burns to place numerous payloads into different, circularized orbits. It opens up significantly more orbital options, particularly for rideshare customers that have traditionally been limited to the primary payload’s designated orbit. Powered by Rocket Lab's 3D printed liquid propellant Curie engine capable of 120N of thrust and multiple burns.

From the images, it looks like the stage fits underneath the payload adapter. According to the news article you linked to, the primary payload was deployed, then the stage+payload adapter+secondary payloads flew to the deployment location for the secondary payloads.

Propellant is "a green mono-propellant stored in four spherical tanks", but composition is unknown at this point.

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    Assuming an OD of 1.2 meters, I'm ballparking those spheres at 16cm diameter and 2kg each. At an Isp of 250 seconds all four would give a 150 kg payload about 130 m/s which is double what would be needed to circularize a 200x500km orbit. So it all seems quite plausible. Thanks! – uhoh Jan 29 '18 at 17:39