Id like to add a few points.
In a universe without a god, ruled only by natural laws, free will most likely cannot exist. Since every particle is ruled by set laws of nature, even something as complex as a human being is just a composition of such, and is therefore ruled in a predetermined way by those same laws. The whole course of the universe will in the same way be set from the very start (if a word such as start even applies to a universe, who knows). We could never calculate or verify this, due to the immensity of the factors affecting said things.
(It is possible that a free will system can appear by nature, trough evolution, who can be sure.. the concept is very hard to grasp. A simple example to think in the right direction about this is an electric relay.
http://www.allaboutcircuits.com/vol_4/chpt_5/1.html
An electric relay is a very simple way to create one system that is controlled by another, where the second system is quite independent of physical laws, in the sense that it can for example be made of a vast amount of different materials and work in the exact same way regardless of them, thereby partly taking natural laws out of play.
IF our brains, or some electronic brain we will some day create, are/can indeed be independent at least partly from natural laws, then free will can exist (without a god).. id greatly appreciate any input on this paragraph, in pm if you feel it wouldnt be on topic. As an afterthought, wouldnt it be ironic if a computer we one day create will be the first example of free will?)
Now, if you accept my first paragraph to be true, at least theoretically, in a universe that DOES have a god, this means that it must be omnipotent (omniscience is not an issue, though an omnipotent being should be able to acheive omniscience at will, or lose it.) because it will have to alter the rules of nature (which we claim evidently exist) in order to give any being the gift of free will.
So, to sum it up, a god probably needs to exist in order for free will to do so, and it must most likely be omnipotent, or at least be able to manipulate life in mysterious ways.