We have developed a Navier-Stokes solver, called cuIBM, to simulate incompressible flows using immersed boundary methods.  This document provides background on the numerical methods implemented in the cuIBM code framework and evidence of the validation exercise carried out by the authors. The code provides a growing set of options for flows with immersed boundaries, is written in C++ and uses GPU hardware via CUDA kernels and calls to the Cusp library. The validation tests in two-dimensions use an analytical solution (Couette flow) and several experimental results to contrast with the numerical solutions. The experimental benchmarks include a lid-driven cavity at Re=100, impulsively started flow around a circular cylinder at low and moderate Reynolds numbers, cylinder wake flow, and flow over both heaving and flapping airfoils.