Can I cross the US/Canadian border with a copy of my passport? We left originals at our house in Florida because we didn't think we'd be going to Canada, but we'd like to go to Niagara Falls for a day.
The short answer: NO
The long answer: To enter Canada from the USA as a US citizen you need to produce the actual passport or an enhanced drivers license or a standard drivers license plus birth certificate.
The return to the USA you need your passport or enhanced drivers license.
Photocopies of passports are not legal documents for crossing a border, as they could be easily forged.
Florida does not issue an enhanced drivers license yet, even though they do now require you to prove yourself to be a legal resident of the states.
You are pretty much limited to viewing the falls from the US side.