I know sky scanner, and find it very useful. But I wonder if there is some similar website, where you can register, and add some flight alerts, so the system continuously monitors prices for your flights, and when it finds something cheaper, send you an email.
I frequently use Kayak, for North American origin flights. I've found that it works very well for flight alerts, although it does require you to sign up first.
I like Sky Scanner as well - especially for European vacations, when I just want to fly anywhere and use Ryan Air or other cheap fairs without caring about my destination - but I've found it more difficult to use in North American origins.