Magnetic field effects have been measured in various processes involving radical pairsand such magnetic field effects have provided the basis for a popular hypothesis of magnetoreception in migratory song birds. The spin dynamics of radical pairs exposed to radiofrequency radiation have also been associated with changes in the production of reactive oxygen species within a biological cell, an effect that is potentially harmful. In order to investigate such phenomena theoretically, one needs to employ complex computations which rely on solving stochastic differential equations, which may appear significantly different for slightly different problems relying on spin dynamics. To avoid creating a manifold of specialized tools for various spin chemistry problems, MolSpin has been crafted as general software to handle spin systems of varied complexity. In particular, it permits solving the Liouville-von Neumann equation to obtain the time-evolution of the spin density operator, calculate quantum yields allows us to utilize semiclassical methods, enables calculation of energy levels in an arbitrary spin system, and predicts resonance frequencies. MolSpin is designed with a high emphasis on extensibility that makes it easy to implement new or extend existing functionality.