In most states, the deadline to register to vote by mail has recently passed (see http://www.eac.gov/voter/docs/state-reg-deadlines.xls/attachment_download/file) but the deadline to register is yet to come in Alabama, California, Connecticut, Delaware, Idaho* (mail-in today!), Iowa*, Kansas, Maine*, Maryland, Massachusetts, Minnesota*, Nebraska, Nevada (mail-in has passed but you can still walk in and register), New Hampshire*, New Jersey, New York (today!), North Carolina (mail-in today or at one-stop stations until Nov 1), Oklahoma (mail-in today!), Oregon, South Dakota, Utah (walk-in), Vermont, Washington (walk-in), West Virginia, Wisconsin*, and Guam.
*in starred states you can also register on election day, if you miss the mail-in deadline, which is also true for Montana and Wyoming, whose mail-in deadline has passed. North Dakota does not have voter registration, according to that reference, so I guess North Dakotans just walk in and vote.
Do register, and encourage others around you (or in states mentioned above) to do the same.