Priceline Group: priceline.com, agoda.com, booking.com, cheapflights.com, ctrip.com, KAYAK, Momondo.com, Opentable.com, rentalcars.com, Rocketmiles
Expedia, Inc.: Expedia.com, Carrentals.com, Classic Vacations, eLong.com, HomeAway, Hotels.com, Hotwire, Orbitz Worldwide, Travelocity, Traveldoo, Trivago, Venere, Wotif Group
In case of Priceline Group, it depends on the market. E.g. in China Agoda seems to be just another front-end for booking.com. All bookings are actually made through booking.com. But e.g. in Malaysia Agoda lists some Hotels which I could not find on Booking.com.
The Flight search engines seem to have major differences, e.g. in Southeast Asia Momondo lists a lot more local agents and more airline websites directly than Kayak.
I am a bit shocked though, that Momondo is owned by Kayak. For me, Momondo has so far been the best alternative. And Skyscanner is owned by Ctrip, which has a strategic partnership with the priceline group...