  Probing the boundary between classical and quantum mechanics has been one of
the central themes in modern physics. Recently, experiments to precisely
measure the force acting on milligram scale oscillators with optical cavities
are attracting interest as promising tools to test quantum mechanics,
decoherence mechanisms, and gravitational physics. In this paper, we review the
present status of experiments using milligram scale optomechanical systems. We
compare the feasibility of reaching the quantum regime with a pendulum, torsion
pendulum, and optically levitated mirror. Considerations for designing a high
$Q$ pendulum, condition for torsion pendulums to have better force sensitivity
than pendulums, and constraints in designing optical levitation of a mirror are
presented.
