Carticipate is a great app that links you up to social network of folks who want to share rides. You can hitch a ride with them, or open up your vehicle to share a ride. While limited right now to the small number of users, it is growing and will hopefully soon be a catch-all for ride sharing.
With this app, you can plug in where you’re going and find others in your social network who are headed in the same direction.
Check out the iTunes product page for more details.