I’ve used Sky scanner for around 5 years, I’ve booked major flights like LA-Melb. To Lon-Melb with them. It doesn’t get much better than this website when it comes to comparing and booking your flights.
There is a new website from google recently unleashed, aimed to be better then travel agents!
I managed to find a mega cheap return flight o Barcelona via google flights. It is very up to date with flash sales and everything where as sky scanner can be a bit delayed with updating. Google flights seems to be the way of the future, as there is no more powerful search engine in the world then google.
See also How to get cheap flights