I feel that a basic multiple civics test would not be out of line. (E.G. How many branches of government are there, and what purpose do they serve?)
If people are going to vote, they should at least have an idea of how the government works. Knowing who the President, VP, and Speaker of the House are wouldn't be a bad prerequisite either.
An even better one would be proof of identification, but that would eliminate too much voter fraud.