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Before 2010, the ATM was the best source for least amount of fees - was easy to find a bank with less than 1% tacked on fees.

No more - now just like credit cards, most banks typically charge 2-3% "currency conversion fee". But - some cards are better than others.

ATM: Limits are usually $500 or $1000 and this depends both on the bank owning the ATM machine as well as the bank where the account is kept.

Bank of America ATM holders - use the Scotiabank ATM machines -- they have a lowest fee of 1% (before 2010, there was no fee involved). Downtown Montreal Scotiabank ABM/ATMs map. Even with this fee, the exchange rate used by these banks was a percent or so less than those used by two other banks I used (tried all three on a single day). The other banks were a local credit union, and an internet-only bank. Surprisingly, the big Bank of America turns out a better deal for foreign ATM transactions (as long as Bank of America has a special deal with the foreign bank).
Visit the Bank of America site for names of countries and banks that can be used. Note that using a bank not in that network will result in exorbitant fees, in which case this bank is best avoided.
Another thing to keep in mind - if your ATM pin code is over 4 digits, it may be rejected -- in which case, try only the first four digits of your PIN code, that usually works.

Credit Cards: The Capital One credit cards do not yet charge any extra fees for credit card transactions. Now that the free ride on ATMs is history, credit cards may be a good deal even for smaller transactions of $20 or more, certainly credit cards are now the better choice for larger transactions such as hotel stays - no need to lug around too much cash. Other than Capital One, all other credit cards seem to charge at least 3% in fees.

See Foreign Exchange Fees Going Up for more details on both ATM and credit cards.

Cash: In Montreal, with a bit of shopping around, one can find very good exchange rates at some places. Easy to find some that beat the 3% charges of ATM and credit cards.

After having looked around at many exchange shops - many are on Ste Catherine, 4-5 blocks on either site of McGill street. Prices vary a lot - but no need to look around too much. There is an easy trick to find a if a specific exchange house has best rate - look at the spread between their posted buy and sell rate for a single currency, say USD. If the spread is 2% or close, that is an excellent rate for converting your USD to CAD. Some big exchange shops have spreads of 6-8% - that is an indication of costly exchange. Just walk a block or two more, there will be another place with better rates. If in the Chinatown area, try 1112 St Laurent just below Rene Levesque. The rate is very good (nearly as good as the BofA ATM card mentioned above), and commission is the standard CAD$2.50. Of course, if using the BofA ATM card to get cash, and using a good credit card like Capital One for large transactions, there may be no need to get USD changed into CAD.

In any case, for the infrequent traveler, any choice is fine - even in the worst case you lose just 2-3 cents per dollar, so exchange cash (comparison shop before settling on a place), or use a credit card (no fee for Capital One, 3% for nearly all others), or use a ATM (1-3% fees) - whatever is more convenient.