PICIN: a particle-in-cell solver for incompressible free surface flows with two-way fluid-solid coupling. This paper details a novel numerical approach for solution of the Navier-Stokes equations for free surface flows involving two-way fluid-solid interaction in arbitrary domains. The approach, which is hybrid Eulerian Lagrangian in nature, is based on the full particle particle-in-cell (PIC) method applied to incompressible flows. An extension of the distributed Lagrange multiplier (DLM) technique proposed by Patankar et al. [Int. J. Multiphase Flow, 26 (2000), pp. 1509--1524] is employed for the two-way fluid-solid coupling. The resulting code is called PICIN. Solid bodies can be mobile, either having prescribed motion or moving as a consequence of interaction with the fluid. Numerical results for three distinct example applications of the model in two spatial dimensions are given. A comparison of PICIN predictions with state-of-the-art numerical results of other researchers is made for each of the test cases presented.