Given the cost of gas and other means of transportation, rideshares allow the opportunity to save costs on long-distance travel. Several services are linked below, evaluate each on its own merits. (NOTE: Several allow you to specify what conditions you'd like, for instance all women in the car, etc.)
http://www.erideshare.com/
http://www.ridecheck.com/ridecheck/
http://www.rideboard.com/
http://www.shareyourride.net/
https://www.nuride.com/nuride/main/m
http://www.rideshareonline.com/
http://www.rideshare.us/
http://kindrideshare.net/rideshare.h
http://rideshare.meetup.com/