Sensitivity is crucial for zero-field NMR spectroscopy, especially, when used for on-line materials characterization in processing applications. Anti-ferromagnetic NMR, nuclear quadrupole resonance (NQR) and ferromagnetic NMR are useful techniques to characterize minerals, pharmaceuticals, and explosives in bulk volumes without applying external static magnetic fields. We discuss several signal amplification methods including population transfer techniques, the intrinsic signal enhancement in ferromagnetic materials, circular polarized RF fields, and magnetic-field induced T2 relaxation times enhancements. Full quadrupolar Hamiltonian simulations are used to determine zero-field NMR spectra and to predict signal intensities and amplification factors in single crystals and powders.