I had always thought the Volt was a series hybrid, but I just checked [0] and it turns out there's a mechanical bypass mechanism that can connect the gasoline motor to the drivetrain, under control of the engine computer. Sounds like at low speeds it operates as a series hybrid, but at highway speeds, once the batteries are drained, it turns into a parallel hybrid. Clever!
EDITED to add: dwhall posted a link with much more detail [1] -- very interesting!