Cottrell is one of a growing number of Chicago-area drivers who rent out their cars, or rent others' cars, through peer-to-peer sharing services. The services, which have exploded in popularity in the region over the last year, offer a low-cost alternative to rental companies like Hertz and Budget, or car-share businesses like Zipcar. Instead of renting a car that is part of a company fleet, you borrow one from a private owner.