The scope of my presentation covers my experiences on THz FEL oscillator, superradiant FEL, and FEL amplifier. I will start my talk on the ultra-compact far-infrared FEL oscillator that I built at Stanford University when I was a graduate student there. This FEL oscillator employs a MeV thermionic gun as its injector, emitting far-IR radiation at 85 microns from a hole-coupled cavity. In the 2nd part of my talk, I will then present our recent study on THz superradiant FEL to achieve MW peak radiation power without a laser oscillator. Among several schemes to generate THz bunched electron beams for the superradiant FEL, laser-gated and mask-grated photoemission from a photocathode are my emphases. For applications demanding spectral and mode purity, a laser amplifier is often superior to an oscillator. Recently kW and sub-kW THz seed sources are becoming available from optical technologies. I will report our recent effort in designing a high peak- and average-power THz FEL amplifier with tunablity between 0.5~7 THz. If time allows, I might also talk about our on-going effort on the realization of table-top far-infrared FEL driven by a laser modulated keV electron beam.