Craigslist.org has a category called "Rideshare." Postings on this range from typical ongoing carpools ("wanting to split gas costs with someone weekdays between Oakland and Stanford campus") to single trips ("looking for a ride to L.A. this weekend"). In the San Francisco Bay Area, respected all-purpose transportation site 511.org has a carpool sign-up page at http://rideshare.511.org/ridematch/.