The free boson model is the simplest possible frustration-free quantum many-body system - i.e., all elementary components of the Hamiltonian are minimized by the same ground states. By applying a systematic generalized Witten's conjugation method to this model, we obtain many known solvable models for interacting bosons and their natural generalizations; extending the method, we also obtain models for other interesting states like supersolid, Bose metal with Bose surface, and Bose-Einstein condensate subject to Gutzwiller projection.