I interviewed at a travel company last year. It was interesting because they didn't do any of the backend booking stuff (for flights), but instead seemed to like to direct you to other sites to book and then take a referal cut if you buy. This was a weird business where the person buying the travel was the product (I didn't like it).
Expedia is one of the direct bookers. I use them. The UI is not pretty but functional enough.
I think Hipmunk (the site founded by some of the Reddit founders) works like that. It's not unusual though. Here in the UK, things like insurance and utilities are increasingly sold in this way (we call them "price comparison" sites). I believe(?) Shazam also makes most of its money by referrals to iTunes and Amazon.
that is how kayak and tripadvisor make most of their money too. Ever notice how photo galleries for hotels on tripadvisor open new windows? they're making sure you have their cookie when you book it on hotels.com, expedia, booking.com, etc later.
Expedia is one of the direct bookers. I use them. The UI is not pretty but functional enough.