  We present a superconducting circuit device that provides active, on-demand,
tunable dissipation on a target mode of the electromagnetic field. Our device
is based on a tunable "dissipator" that can be made lossy when tuned into
resonance with a broadband filter mode. When driven parametrically, this
dissipator induces loss on any mode coupled to it with energy detuning equal to
the drive frequency. We demonstrate the use of this device to reset a
superconducting qubit's readout cavity after a measurement, removing photons
with a characteristic rate above $50\ \mu\mathrm{s}^{-1}$. We also demonstrate
that the dissipation can be driven constantly to simultaneously damp and cool
the cavity, effectively eliminating thermal photon fluctuations as a relevant
decoherence channel. Our results demonstrate the utility of our device as a
modular tool for environmental engineering and entropy removal in circuit QED.
