If you’re a Canadian citizen, you don’t need a visa, according the US Consular office website.
Quote from the site:
If you are a Canadian citizen you do not require a visa to enter the United States. Simply take your documentation and proof of Citizenship and proceed to any U.S. Port of Entry.
From Department of Homeland Security (man, I hate that name):
Canadian Citizens – Beginning June 1, 2009
Canadian citizens will need to present a single document that complies with the Western Hemisphere Travel Initiative (WHTI). See www.getyouhome.gov for more information.
US-VISIT biometric procedures may apply to some Canadian citizens. Find out if US-VISIT applies to you.