From LifeHacker, one of my favorite blogs come this suggestion for finding people to share car trips with. The passenger agrees a fare which is effectively held on escrow by Ridester, providing security on both sides.
Subsidies? Nil. Government involvement? Nil. Security? Pretty good, like eBay, you get to read testimonies from previous travel sharers and give ratings.
Two consequences I like: 1) this will tend to reduce pressure for public transport subsidies in remote parts of the USA and provides an ethical alternative to regulated car sharing schemes; and 2) it encourages people to behave responsibly (assholes will be reported as such and lose future opportunities, whether they drive or are passengers).