Every time there's an election it seems like someone brings up the odd fact that you are not asked for ID here in Massachusetts before being allowed to cast your ballot. It seems to me that this is especially surprising to people from other countries. Here's a good article that explores some of the reasons and arguments for and against requiring identification at the polling place.
The gist of it is here:
"In many states, an ID is required to vote. The ostensible purpose is to prevent people from casting a ballot for someone else – dead or alive. Historically, it was also used to prevent poor and minority voters, who are less likely to have government IDs, from voting.
No one would (publicly) admit to the second goal today, so the first is always the declared purpose. But does it work?
In my experience as a pollworker in Virginia, the answer is clearly “no”. There are two basic problems – the rules for acceptable IDs are so broad (so as to avoid disenfranchisement) as to be useless, and pollworkers are given no training as to how to verify an ID." (from Do Photo IDs help prevent vote fraud?)