I learn from Chris Dillow that the question I asked about voting - "Is the fact that others' votes are correlated with mine something I need to take into account when estimating the effect of my vote?" - is in fact a long-standing question with a name: Newcomb's Paradox
The questions are not quite identical - Newcomb postulates an entity called "the Predictor", whereas I am working from the observed fact that opinion polls more or less work.
The question may come down to why Fred Bloggs' vote is correlated with mine. I used a thought experiment in which Fred and I are identical robots being fed identical inputs, and our votes are correlated 100%. In that case, the correlation is due to the fact that the two votes are determined by the same inputs.
On the subject of free will, I take the view that what matters is whether my decisions are all determined by the world outside myself - and I think it's pretty obvious that they usually aren't, and that therefore I am free.
The fact that my actions are determined by the state of the world including myself is both trivial and uninteresting.
The bit that is interesting is "what am I". The relevant answer is that I am a phenomenon of matter - that what I refer to above as the world outside myself must necessarily not include my brain and body. The reason this subject has caused confusion historically is that there was an assumption that my body was external to my self.
(There is much more to the answer than that, but that is the part that is relevant to questions of determinism and freedom).
Scott Adams is interesting on this, though he doesn't yet get the point. I suspect he eventually will.