Skimming through the comments, I want to clarify how the US does things. I don't think we're sending a clear picture. In the US, voting rights are determined by state. In some states (12 of the 50), former felons cannot vote depending on the crime. In most states, ex-felons can vote after getting out and maybe after a probationary period. In Maine and Vermont, felons can even vote from prison. However, this system is complicated, and when people move from state to state it is up to the states to keep track. Many felons manage to vote despite not being legally able to, its very hard to document all of this.
Should felons get to vote? It depends how you view the State and the criminal justice system. Some view justice as punishment: by breaking the law and harming citizens you lose rights like the right to vote. Others view justice as defensive and utilitarian: criminals go to jail to protect the law abiding, and they get out once rehabilitated. Rehabilitated individuals seem like good citizens which deserve a vote.
My take? Those in jail shouldn't get to vote. It requires using state resources to facilitate. But once out, it takes a lot of effort for US states to prevent them from voting, and it frequently goes wrong (voter fraud). Not only is it easier to just let all free men vote, I think its fairer. Don't former felons (many, many of whom are minorities) contribute to society's discussion on our justice system, laws, and politics? I think so.