Apps and tools including Turbo Vote, Voting Information Project, Ballot Scout, and the National Association of State Election Directors give information about what's on the ballot, how to register to vote, and can track a mail-in ballot.
State-by-state chart of voters' rights. Refer to this if you have to work on election day and want to know your rights to leave work in order to go vote.
With Millennials as its target audience, Rock the Vote emphasizes the importance of getting the nation's youngest eligible voters registered and informed.
In addition to all sorts of other helpful info on the election process, this nonpartisan site has a helpful "polling place locator" component. If you don't know where to go on election day, check out this link.
From the site: "Americans are eager to adopt digital voting solutions. Vote.org has built and will continue to iterate the digital technology that removes barriers to voting and increases voter turnout."