New tickets as of 2008.
It costs CAD$7 to buy the OPUS Card. It is a micro-chip based card, and it is necessary if you wish to buy weekly or monthly passes. OPUS can store multiple single tickets on it also.
The à la carte tickets are paper-based with a magnetic strip. These have no additional cost other than amount of fare.
There are some minor disadvantages with the OPUS card for tourists:
1) Unlike other cities (Boston for example), Montreal is charging a fee to buy the OPUS card.
2) The OPUS card has an expiry date - which is four years. This means that every four years a new card has to be obtained. If your old card has not expired for less than 6 months, you can get a replacement for free at the Berri-UQAM office.
3) Unlike many other cities' cards, the OPUS card does not store cash, so it cannot be used for multiple passengers.
For tourists visiting for multiple weeks, there is no choice but to get a OPUS card since the weekly pass (Mon-Sun validity) will not be available in paper-ticket form. Monthly pass is also only available on OPUS.
For tourists who need only 1 or 3 days passes, or only evening/weekend tickets, there is no need to get OPUS card and you can just buy paper tickets.
One problem for US tourists is that credit cards will not work in the automated machines. While the STM web site claims credit cards are allowed, a bunch of cards I tried were all denied. It was later confirmed in a newspaper report that STM has disabled use of non-Canadian credit cards.
On the bus, only coins are accepted, and only single tickets can be bought.
One very good about about STM fares is that they have many weeknight and weekend options, which are all incredible deals for tourists. These don't require a OPUS card, and can be purchased as paper tickets at the same price.