CarPoolWorld
Matches commuters based on proximity to home and work. Worldwide.
http://www.carpoolworld.com/

Carpoolconnect.com
A commuters search engine. Enter a zip or postal code and find commuters in your area searching for you to share a ride.
http://www.carpoolconnect.com

Freewheelers
Lift sharing database connects drivers and passengers to share the cost of travel to work, festivals, gigs and sporting events.
http://www.freewheelers.co.uk

AlterNetRides
Free service nationwide that matches up commuters. Includes links to other resources.
http://AlterNetRides.com

Drive2Day
Cost free carpooling service which offers carpools world wide. Search for drivers or passengers.
http://www.drive2day.com/

Lift Share
The online journey matching service that finds you travelling companions.
http://www.liftshare.org/

Compartir
Carpooling via a geographic search engine. Enter your requirements.
http://www.compartir.org/

Ridester
Feedback-rated system for one-time travel.
http://www.ridester.com/

Carpool Crew
Includes search by radio station and interests to match up commuters with similar interests.
http://www.carpoolcrew.com/

GishiGo
Matches travelers for one-time trips.
http://www.gishigo.com/