Best sites for cheap flights?

No single site books every airline; no site I have found is consistently cheaper. Usual advice to this common question is to research several sites and then go directly to the airline for quotes. Sites such as www.kayak.com aggregate other agency sites while www.cheapoair, www.orbitz.com, www.hipmonk.com and many others can combine several airlines for a quote. Factors to consider: Duration of the flights; connecting times (very important) and number of connections; and for me, no getting up before dawn to catch an early-morning flight. Price comes before all; frequent-flyer points are a less critical consideration. One thing in your favour is that trans-Atlantic prices rise and fall with the thermometer so you should pay less than a traveller in June.
Canadian charter fares are a separate question which you need to Google. At that time I suspect Air Transat and the rest are steering to the sunshine destinations.

They may have already come up in aggregate searches you've done, but check out Icelandair. Their fares are consistently cheaper than anything I've found, and the quality of the airline is great unless you want a direct flight (all of their flights have a short layover in Reykjavik).

I use kayak.com religiously. They don't sell tickets but can alert you to the low fares. You click through to buy from the actual carrier. DO NOT use cheapoair. They tried to scam me and I've since read many times that others had the same experience: http://www.tripadvisor.com/ShowTopic-g1-i10702-k3794547-Warning_Cheapoair_com_is_a_scam-Air_Travel.html

I start at Kayak, but always check airlines directly before I book. There's also a good site for European flights. I travel only short legs each day, so I never use it (and can't recall it's name). For a long leg, you need to use a search engine that specializes on cheap European flights.

I've now booked my flights - after much comparing I did choose to book online thru Air Canada - from/to Nanaimo. Comparing any other flights/locations still seemed to be more once I factored in getting to another airport. Thanks for all your suggestions...will remember everything for a future trip!