The National Association of Secretaries of State run CanIVote.org, a website allowing you to look up whether you are registered to vote in a particular state. This website links users to the appropriate page for each state so they can find out whether they are registered in that state.
Each state is in charge of its own website, so the information you must provide to find out if you are registered can vary from state to state. Typically, a user must provide his name and birth date in order to access information regarding voter registration.
HeadCount.org also provides a list with links to each state's voter registration website along with a phone number for each state's voter affairs office.