The best free alternative to LTspice is Circuit Simulator, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to LTspice and 19 is free so hopefully you can find a suitable replacement. Other interesting free alternatives to LTspice are QUCS (Free, Open Source), Xcos (Free, Open Source), OpenModelica (Free, Open Source) and Ktechlab (Free, Open Source).
StochSD (Stochastic System Dynamics) is a System Dynamics simulation software with focus on stochastics. It is a form of Continuous System Simulation (CSS) that in addition to model and simulate continuous flows between compartments, can handle transitions of discrete entities.