If you are trying to piece together an itinerary using one-way tickets you are inviting lots of grief, at least crossing the Atlantic. It is a good idea to fly into one city and return home from another but to find decent prices you need to search for a round-trip itinerary use the "multi-city" or multi-destination search function on Air Canada/WestJet/Air Transat or whatever.
The most comprehensive source of information is www.itasoftware.com run by Google for airlines. It doesn't sell tickets but the information can be useful. No one site covers all flights and the easiest source for European short-hop budget flights is www.skyscanner.com These low-cost airlines operate on one-way tickets rather than round trips and most of them do not connect to flights on other carriers, and often don't even connect to their own flights. But they can be very cheap. And they don't appear on North American-oriented websites.
For the intercontinental part of your itinerary, Cheapoair.com, airfarewatchdog.com and other search sites do offer notifications but you are better off signing up with the individual airlines. And don't bet on there being sales that suit your itinerary. Prices appear to be edging down a little bit compared to last summer trans-Atlantic but the amount of worry and stress involved in waiting and watching to save $100 is not worth it. For the European budget lines, prices only go up so buy as early as possible.