I'm staying in the US under the Visa Waiver Program at the moment, until 8th March. I'm planning to visit Montreal, Canada, for a week and then come back to the States.
Is a new visa waiver required to come back to the States or is the old one still valid after leaving the country?
If you enter the US under Visa Waiver, then exit to Canada and reenter the US you do not need a new Visa Waiver. More importantly, for calculating the maximum length of stay in the US you are treated as not having left the US for the time you were in Canada. You will need to leave 90 days after the first day you arrived.
Your ESTA is valid for 2 years, provided your passport doesn't expire sooner.
If you return to the US before your original 90-day period expires (i.e. before the handwritten date on your US entry stamp) then you won't need an I-94W. Otherwise, you will need it, but with a valid ESTA they'll print a pre-filled form rather than you having to fill it out manually.
Going to Canada is not supposed to reset the 90 days on the ESTA. However, in my case my (pedestrian) exit at Rainbow Bridge was recorded as a full exit, and at Toronto preclearance, getting back into the USA (by flying to Chicago), I did get a full new 90 days. This did confuse the official but the computer had already approved it so that's what I got.
