I would highly recommend both hotwire and priceline, flycheapabroad, and bookairlinetickets.com. I've searched around for airfare quite a bit, and these three sites usually ring true as the cheapest. Other than that, I'd go to sites like igougo.com, travelzoo.com, kayak.com, and bookingbuddy.com. These sites offer searches of multiple online airfare websites, making it easier and less time consuming to compare prices. As for when to buy, I usually go to farecast.com. They pretty accurately predict when a good time to buy a flight would be based off of past and current trends, etc. It's pretty useful. Another site I've found handy is rightflight.org. They did a bunch of research on the various airfare sites to see which sites are usually the cheapest. They also have a list of most of the international airfare sites on their webpage (60+), which can be pretty handy in and of itself. Hope this helped!

Typically, airlines will load new fares late in the evening, but there are instances where sales can occur that last several hours at the most, so the price could well change during the middle of the day. Add in the complexities of airline pricing, which price tickets based on available inventory on a specific flights, and you'll find that tickets can change on a nearly constant basis.

Ultimately, what this means, in my opinion, is that there really isn't a best time of the day or week to buy cheap international airline tickets, because of the way that pricing works. Additionally, there are quite a few other factors that go into determining the price for international destinations, such as demand of the route and destination, the minimum stay, and so on. The page linked below has some discussion on some of the things you can do to improve your chances of finding a cheaper ticket.

Virgin Atlantic don't serve Canada. Try WestJet for international flights within North America, and try Canadian Affair for flights to the UK. These do not show up anywhere else, not even on aggregators such as Kayak, or mega-aggregators such as http://www.bookingbuddy.com which not only search Kayak, but even more aggregators, plus consolidators, and actual airline websites.

Sure, as long as each place will let you enter on a one-way ticket without showing proof of onward travel. Many places won't. You can check that here: http://www.delta.com/planning_reservations/plan_flight/international_travel_information/visa_passport_information/index.jsp

Are you aware of Tiger Air, Jetstar, and other low-cost airlines? Here: http://www.discountairfares.com/lcostaus.htm It might be cheaper to book your own one-way flights with these, and use your printed e-ticket receipt as "proof of onward travel" when entering each country.

The big airline search sites usually don't cover these new low-cost airlines. However, it wouldn't hurt to try your itinerary with these multi-stop searches:
http://matrix.itasoftware.com/cvg/dispatch/prego/multi
http://www.kayak.com/flights?mc=y
http://www.mobissimo.com/search_airfare_multi.php
http://www.airtreks.com
and http://www.statravel.com.au/cps/rde/xchg/au_division_web_live/