Free will is an interesting topic that you don't see many atheists talking about when arguing against the validity of the god of the Bible. However, I personally, used it as a springboard into atheism and think that others should learn a little bit about the lack of existence of free will. Let me explain.
Free will does not exist if people cannot act and could not have acted any other way than they do. However, free will appears to exist because there is an illusion that we could realistically make different choices when in the same situation under the same circumstances. For example, if I say don't think about a "purple polka-dotted elephant" you will inevitably have to think about it, if only to acknowledge that you are not thinking about it. In this case, you do have a "choice" to think about the elephant or not, but I have caused you to think about it.
Surely, you wouldn't think it was your will to think about the elephant, so let's look at a separate example. You are craving an ice cream cone, so you head down to your local baskin robins. Here there ought to be 31 things to choose from, yet what you choose is still determined. How so? Well, because you like the taste of one flavor over that of the other flavors. You buy your favorite because it gives you the most pleasure, but the fact that it is your favorite is not your choice. Your taste buds and brain have already determined that for you. In this case you are acting how you "want" to act, but the desire you have for, let's say chocolate, was not selected by you.
Well, what about choices not determined by our physical configurations? Think about why people in the Middle East are more likely to be Muslim than people in the US. Do you agree that we are a product of our experiences? None of us have chosen where we were born, to what parents, and what experiences we had thrust upon us. (Just skimming the surface on this one.)
I believe that people will ALWAYS act as they 'want' which is the way that provides the most pleasure/benefit and the least pain/suffering. Choosing physical pain for the benefit of others, such as giving blood, fits into this explanation. It does so because the person experiences emotional pleasure, among other things, that outweighs the temporary pain. Because there is always a specific choice that will be made in a given set of circumstances, a person never truly has an option to act differently.
I know this probably isn't all encompassing, and I didn't use the words cause and effect, but I'd rather discuss questions than try to cover everything I can think of. Looking forward to the discussion!