This is the homepage for Skiplagged.

It uses a trick it calls "hidden city ticketing". As The Telegraph explained:

The idea is that travellers wanting to fly from Dallas to Los Angeles, for example, are instructed to book a flight to an alternative destination, say San Francisco, with a stopover in Los Angeles. They then don't bother to take the last leg of their journey.

This isn't always the cheapest way to buy tickets, but it often works.

They claim it's creating "unfair competition".

Fox 31 Denver reported that the website is a "side project" for Zaman, who "grew up in Brooklyn, N.Y., and graduated with a bachelor's degree in computer science at age 20 from Rensselaer Polytechnic Institute". He claims not to have made any money from the project.

Zaman told the network that a lawsuit was "inevitable", but he says there's nothing illegal about his website: "[Hidden city ticketing has] been around for a while, it just hasn't been very accessible to consumers."