Based on parallel-plate waveguides (PWWGs), we have developed both passive and active terahertz (THz) sensors, filters, splitters, and switches. We utilize the PPWG since it has excellent waveguiding properties with a simple geometry that allows for incorporation of unique electromagnetic functionalities. A few passive devices are discussed while the focus remains on active devices. The active control is enabled by our innovation to incorporate liquid metals, which can be relocated by application of a small electrical voltage (< 4V), as part of the waveguide geometry itself. The device geometry directly affects the device performance (e.g., frequency band of operation, channel output power). Therefore, by moving the liquid metal, the geometry is reconfigured which results in altering the device function. To show the practicality of these devices to be used for THz wireless communications, we demonstrate the ability of these devices to support a 1 Gb/s data stream on a THz carrier wave and show that this signal can be successfully switched with high modulation depths of ±40 dB. These results show the strong promise of these components to form important signal processing building blocks in the future infrastructure of THz wireless communications.