Most simulator do have a Monte Carlo option and what they basically do is to run a simulation (dc, transient, ac, etc) a number of times to give you a set of values or family of waveforms. Each time a simulation is run, a new set of component values are selected and the simulation is run. The selection of component values depends on what type of distribution you assign to that value. For example, you can choose the resistance of a resistance to have uniform distribution -- distributed equally from 8-12k where 10k is the nominal value. Or you could have a normal (Gaussian) distribution ....