The Obama campaign in 2012 had a smartphone/tablet app that they used in the early days of the re-election campaign to register voters as part of their first canvassing passes. Being open-source software, any person or organization that wants to help increase voter turnout could modify it to suit their specific needs - maybe they rely on older voters, so they want to make a more high-contrast version, or they need to translate the app for their voting bloc.

You could also work to make an API out of the state data (it's a a ton of work to aggregate all that), or work to make the site an offline application so that voter registration forms can be created, saved, and later emailed, or even printed and snail mailed to voters. The biggest obstacle with voter registration in the US is that you have to physically mail a sheet of paper. I'd be willing to bet that few unregistered voters probably also have a printer, stamps, and an envelope all ready to go.

Now, one caveat to all this is of course that I'm not sure how much of it is completely kosher. There are lots of regulations on this stuff, obviously.