What is the best site or app for cheap flights?

I use hopper for saving money on flights and getting notified when there are price changes.

Hipmunk has a great interface, and their proprietary sorting mechanism (I think it's called Grief) is well done.

Reveal hidden prices. They claim this - "We're better at finding cheap flights than anyone else— so good, United Airlines actually sued us for it". They show hidden city ticketing trips in addition to what sites like Expedia, KAYAK, and Travelocity show. Taking a stop might save hundreds of dollar as @melaniepinola mentioned here - http://lifehacker.com/skiplagged... You can also track previous searches for changes in price. Available on iOS/Android.

I've been using Corner for a long time. Hard to beat them.

Most widely used travel information search platform. Compare the cheapest flights of every airlines. Best flight tickets using month view is cool (if flexible).

Just enter your budget and see where you can travel to, no superfluous bullshit.