Montreal Museum Day is a special annual event where almost every museum in the city is open for free! Also, there are special shuttle buses that run to all the museums. There are about 30 museums participating from 9:00 AM to 6:00 PM. It is crowded, so be patient, but it is a lot of fun to visit so many museums in one day.
You can get a map and more details about which museums are included this year at their website: