He cannot be retried for murder in Florida. He could be sued in civil court by Martins' parents for wrongful death.
He could be tried for civil rights violations by the Department of Justice, but I think they are not going to do this.
As for what his trial and the evidence did and did not "prove", I am at a loss to see how it proved Zimmerman's version of events. It was my reading of the tea leaves and reported evidence that it left significant doubt as to what actually transpired that night. In this type of case, our legal system (or more specifically Florida's somewhat odd burden of proof in a self-defense case) makes it relatively easy to get a not guilty verdict.
I am not sure that this was the right decision on all the charges, but I am comfortable letting the jury decide and respecting their verdict. My opinion of Zimmerman is that he is a reckless dumbass who sooner or later will really **** up.
I hope he doesn't.