I have to vote for the Air Canada website. You have to check the fares all the time, though, because they change each day, and sometimes three or four times each day. Usually, Mondays seem a bad day for bargains, and Wednesday and Sunday nights seem good. Also, because they have a feature showing how many seats are "available" on a flight, you sometimes get an idea which ones will come up for sale in a day or two.
As well, there is a different sale on from day to day, and sometimes the make no sense at all. I bought a flight on AC from Winnipeg to Calgary for $9 this month (of course, it ended up being much more with the taxes and fuel surcharges) because Westjet had the same special. Westjet had all their $9 tickets sold out, but AC had three flights a day in each direction that were still selling for $9 when I bought mine.
The other strange thing is that different departure cities have radically different fares, sometimes. I was looking at Vegas last week, and it was $169 from Ottawa, same thing from Toronto, but $59 from Winnipeg, with the flight through Toronto. Now, I don't know if you could buy that, and just get on in Toronto, but it serves to show the vagaries of the system.
All that being said, you end up putting in a few minutes to a half an hour each day looking for the cheapest fare, which often leaves early in the morning or late at night. You may save $100 to $200, because we're only talking domestic travel, here. It is worth it for me, because I is po', and I find it interesting. For someone with less time on their hands, you just can't save enough this way, because the ticket will only cost maybe $200, with much more being the taxes and charges which are never discounted.