Car sharing is designed to help cut down the number of cars travelling with only one person in them. A bit like organised hitching, the web is the perfect medium for this and there are several sites that put drivers in touch with wheel-less travellers. Usually free to use, you register online and are then put in touch with people who are driving the route you want to cover. Passengers share the cost of the journey with the driver. This site works as a directory of some of those sites.
Like that? Try www.carplus.org.uk