passenger
person who travels in a vehicle without operating it
hitchhiker
one who asks people, usually strangers, for a ride in their road vehicle
Gratis passenger or partially free
Tickets to noncommercial reduction granted by the airlines to their employees, but also their immediate family (spouse, children, parents, partners, etc).
stowaway
unauthorized passenger who secretly boards a vehicle
back-seat driver
passenger who excessively attempts to direct, advise, or criticize the operator of a vehicle