Voting is already underway in seven states, and will begin in Arizona, California and Illinois in the next two weeks. (Incidentally, groups in Illinois launched a "Vote Naked Illinois" campaign to inform voters of changes in voting law that now allow Illinois citizens to vote early.)
The Early Voting Information Center (EVIC) has created a calendar that tracks each state’s window for early voting. See it here.
70% of Americans are now able to take advantage of no-excuse early and absentee voting. Check out this New York Times graphic that breaks down the percentage of ballots cast before Election Day in 2006 and 2008.