Do I need a Canadian passport to travel between the U.S. and Canada?

At the present you only need a Canadian Passport if you go to the USA by flying.
At the land crossings you need proof of Canadian Citizenship only. Probably a birth certificate plus some official photo I.D.
This is supposed to change in 2008 to Passport Required at all crossings.
Should you be a Landed Immigrant (not born in Canada) not a Canadian Citizen, then you must have a Maple Leaf Card. (Official Card you must apply for)Remember you will be subject to interview by Canada when you return.