http://www.carshareforum.co.uk/ Is for free car-sharing arrangements within the UK.
Other than that I'm struggling to think of any travel arrangements that are both legal and completely free - if "cheap" is close enough to free for you, you can try blablacar.com (or your own nation's domain - it redirects me to own nation's domain if I type in .com) for car-sharing across Europe - find a ride, time, and passenger group that suits you. It can be only a few euros/pounds (<5, just petrol costs divided evenly between passengers) for a lot of trips, and it comes on personal recommendation.