We explore guided modes in metallic "spoof-insulator-spoof" (SIS) waveguides: parallel plate structures with subwavelength corrugation on the surfaces of both conductors. A dispersion relation for SIS waveguides is analytically obtained. The modes in the structure arise from the coupling of conventional parallel plate waveguide modes with the localized modes of the grooves. SIS waveguides can be engineered to guide modes with low group velocities and SIS tapers can be used to convert light between photonic modes and plasmonic ones.