Mitfahrgelegenheit (we travel together) is a popular car sharing program in Germany. It works this way - suppose you are traveling with your car from Berlin to Bonn. You have 3 seats to share. You post an offer with a minimum charge (3 Euro per month) in the Mitfahrgelegenheit portal. A potential traveler searches in the portal for someone who's driving to his/her destination from his/her city. The navigation is easy with an easy source to destination and time specific search (with tolerance settings) and all the traveler has to do is to call/email you(no fees to Mitfahrgelegenheit) if yours matches with his/her requirement. You set up the pick up place and payment option. So you can have 3 persons @25 Euro per person (arbitrary) with which you can compensate the fuel expense for the ride. And the ride sharers have the chance to reach Bonn with one fourth cost of a regular railway fare in the cozy seat of your car (music and gossip as extra bonus). The traveler does not have to pay anything to Mitfahrgelegenheit for the search. Its safer than a hitchhike. Because you are not getting into a stranger's car on a deserted highway. You have the driver's name and cell-phone number it is recorded in the internet.
This service was introduced in 1998 and within 5 years have become the market leader. It now has 700,000 register users, average 100000 travel offers and daily 10000 rides carrying average 30000 travelers. They have a twelve member support team to tackle misuse of the system and help not serious users.
In USA there is a similar service called Ride Check but with limited offers because of limited users.
This is a good example of citizens network service and it works fully when more and more players are using it. I once was riding with one of my German friends who frequently hitchhikes. He was dying to pick up people from the street as we had one seat empty and was mentioning, if you are a hitchhiker, you know how does it feel to stay on the road waiting for the opportunities. Ride sharing networks reduces these tensions.
By sharing rides you help others as well as yourself and in the end you doing something good to the nature.