This probably is checked if you want a visa (staying more than 180 days).
I think you can still gain entry into Canada with just a Government issued ID (e.g., driver's license), but check the CBSA website.
In 2006 I made a trip to Algonquin National Park crossing near Buffalo, NY. I got in with just my driver's license. I did not have my passport with me since this was a spur of the moment trip. I did have major problems getting back into the US without a passport, because without a passport they wanted two forms of government issued ID and I only had one. Of course my normally cheerful and patient attitude towards government bureaucracy did not help :)
In 2007 & 2008 I traveled several times to Canada via the airport in Vancover, Canada. Takes a little time to clear customs, mostly due to the lines of people. Again, more difficult and time consuming to leave and get back into the US. But this time I had my passport. If you do not have a passport, apply early as sometimes this can be a timely process.
The rules might be different now.