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/.
To organize a small carpool, contact friends and family nearby and drive to usual areas such as grocery stores and you may want to schedule appointments such as haircuts together and things such as birthday parties at places like movies take several of guests in on of the larger cars or a smaller number of guests in an average sized car. If you feel crammed, remember that you are saving a planet that needs your help and that you are being a hero to our planet.