We study the collective excitation spectrum of a two-dimensional charged Bose gas interacting via long- and short-range potentials. The resulting plasmon dispersions depend on the type of interaction. Extending our results to a double-layer system, we calculate the dispersion relations of ensuing optical and acoustic plasmons, and screened interactions. The effective interactions exhibit attractive parts which may have interesting consequences. Comparison of our results with a two-dimensional electron gas is made.