Scott Adams thinks that free will is an illusion – humans are nothing more than moist robots. This is essentially the Newtonian view of human behavior. If you can discover the right factors and measure them you could predictably guess any individuals actions.
Others (perhaps most) believe that you can do anything and that every choice is voluntary. In this model there is no physics, every human action is random.
There is, however, a third alternative. Genetics, environment and upbringing will limit the possibilities considerably. Humans won’t make completely random choices, they are limited to a finite number of options based on who they are.
I call this the quantum mechanic view of free-will. Individual behavior are pretty tightly bound by external factors – most people will follow pretty predictable behaviors – however, there is some probability that small deviations can take place.