I saw some posts asking for the Bosch LapSim which is of course very good and has some quite complexity to it.
Some time ago I stumbled with OptimumLap:
It's free, it's basic. According to them "OptimumLap is a free, simplified, vehicle simulation tool designed to estimate a vehicle’s performance within 10% of reality."
I have no association with them, just thought it'd be cool to share.
BTW, Optimum's website has a blog with very nice information that is always backed by a lot of data.