Dumpster-diving: The bins need more exploration by people that use this wiki, yet there definitely are good dumpster-diving opportunities. There was an active FNB group that is now waiting for new people to do it. FNB Ottawa
One place where you can surely do a good dive is Rideau Bakery on Rideau street. They throw out different kinds of breads and pastry (including cakes, croissants, muffins, cinnamon rolls... All put in a separate bag and usually in little bags to start with.)
The trashbins of the Market Organics (York street, Market By area) are accessible: they are situated on the parking. However, it seems they are shared with other buildings because you won't find only organic food bags inside. Dumpster-diving there is a real mess because Market Organics do not separate edible food from real garbage, so you'd better wear gloves and clothes you don't mind getting dirty. We once found several large bags of hamburger buns there, probably not from the Market Organics. We once were surprised by policemen patrolling. They asked us what we were doing in the dumpsters, but left us alone afterwards. (last checked:December 2014)
Like most places in Canada, diving in the suburbs is hard because most stores are in fact giant shopping malls with big compressor machines or locked away dumpsters. Closer to the center the situation is much better.
Also, the Salvation Army apparently has an accessible dumpster.
Food banks: If you are indeed traveling without a budget and are hungry, just look for the nearest food bank in Ottawa. They have a central one that delivers stuff to other, smaller food banks. The system is pretty nice and even though you usually need id and proof of an address, if you just say the truth (eg I travel, I'm hungry and I'm staying with a friend), they will help you out! This is the website where you can put in your address and find the closest one: Ottawa Food Banks