My wife stays up nights looking for cheap airfares. 1 Travel, VacationstoGo.com for cruises, Groupon, LowFares.com, Momando.com and DollarFlightClub.com are what she uses. She receives Emails from them often.
I watch ITA Matrix closely, and find the lowest airfares are not necessarily the best value. The time in the air, time sitting in airports and when a flight arrives is also important. I also avoid flying through some airports--like O'Hare and JFK. After a recent flight to London thru Charlotte, we're not Charlotte Airport fans either.
And as experienced international travelers, we seldom pay over 50% of normal airfares. We most often travel before 4/1 or 6/1 or in October and November to get lower flights.