The most common one nowadays is the optical scanner, where voters mark ovals on a paper ballot with their picks and they're read by a machine like those used to score standardized tests. Three out of five counties now have that technology, according to a recent Caltech/MIT Voting Technology Project report.
Most of the remaining counties use touch-screen devices much like a bank ATM, where a computer records votes on a memory card.
A few counties in Idaho still have punch cards. And a small number continue to use hand-counted paper ballots, particularly in lightly populated areas such as the tiny New Hampshire hamlet of Dixville Notch, where 10 voters plan to cast their paper ballots just after midnight in Tuesday's election.
Optical scanners are used in Cuyahoga County, Ohio — the most populous county in that critical battleground state. The county, which includes Cleveland, has about 928,000 registered voters. Cuyahoga switched from electronic touch-screen machines after the secretary of state raised questions in 2007 about security flaws with the system.
The director of the elections board, Jane Platten, is confident her system of paper ballots and optical scanners is solid.
"It's accurate and it's robust enough to handle to dynamics of Cuyahoga County," Platten said in an interview. A big upside with the system, she said, is that it creates a paper trail that can be used in case of a recount or challenge to the outcome.
It's not the voting machines that worry her. "What keeps me up at night is what's going to happen that I cannot predict," she said. "Usually it's something environmental and not system-related."
In March 2008, there was a massive ice storm on presidential primary day. In November 2011, one of her poll workers got in a fight with a voter, head-butted the voter and bit his nose. "Those are the things that keep me awake," she said.
In the nation's largest election jurisdiction, Los Angeles County, officials adopted a system that is similar to the old punch-card routine but instead uses a special ink pen to mark the ballots, which are then read with an optical scanner.
Dean Logan, registrar for the county's 4.7 million registered voters, says the system isn't really different as far as what voters see or experience when they cast their ballots.
"The real improvements that have been made in the last 10 to 12 years have more to do with ballot design, use of plain language in instructions, a greater emphasis on voter education and public engagement" on voter rights, Logan said.
And more and more voters are casting their ballots early, either by mail or in person at central voting centers. In Los Angeles County alone, officials issued about 900,000 vote-by-mail ballots for the 2008 election. This time around, Logan says, the county has already issued more than 1.5 million vote-by-mail ballots.
Elections expert Kimball Brace says many changes took place mid-decade, so many of those voting in the 2008 election had the opportunity to get used to new equipment or design changes. He said the hope for Tuesday's contest between President Barack Obama and Republican challenger Mitt Romney is that voters will be familiar with the equipment and the process at their polling places and that election officials are quick to react to any problems.
"Election officials are always cognizant of changes. The issue always becomes how well they manage it," said Brace, president of the Washington-based consulting firm Election Data Services. "Anytime you have something as close as what we have now, any tiny little thing can throw things off."
That could be a storm, a machine that jams, a computer failure or even an angry poll worker.
Election administrators, Brace said, always pray on election eve for landslide margins. At least in the presidential race, he said, that looks unlikely this year "because it's going to be tight."