Every citizen over 18 should be able to vote, including prisoners.
Prison is punishment in itself. Taking away the right to vote on top of that is unnecessary.
Most prisoners will get out eventually, and the dignity of being able to vote while in prison may help rehabilitation.
Vermont and Maine, though, are the only US states which lets prisoners vote.
In other states, the free people in districts-with-prisons get excess voting power based on people who can't vote being included in the population of their districts.
As the prison population grows every year, the effect of policies towards prisoners and former prisoners on our democracy becomes greater.
For more information,
read this LA Times op-ed ("Let Felons Cast Ballots and Others Will Follow," by Sasha Abramsky, Oct. 31, 2004)
or check out a chart of felon disenfranchisement per 1000 people, with Florida on top and Maine, Massachusetts, and Vermont on the bottom
or watch this Flash Video.