Many airline search engines will also source rental cars. These companies focus on autos:
AutoEurope - specializes in car rental in Europe
Kemwel - specializes in car rental in Europe
RelayRides - peer-to-peer carsharing
GroundLink - order private car service online or by app; worldwide coverage
Uber - order private cars by text or app; in a handful of major U.S. cities and Paris
Limos.com - order private cars, vintage limos, mini-buses, and even trolleys
If you have a favorite site you don't see and want to share, drop us a line.