Casual carpool - the original form of free transit in Oakland.
Casual carpool is a totally self-organized, non-commercial system where people who commute between the East Bay and SF for work in their car pick up people who commute between the East Bay and SF for work but who have no cars at designated spots. The people with the cars get a couple people in their car and get to go across the Bay Bridge faster and cheaper because they can go in the carpool lane for a discounted toll fee (it used to be free to go across the Bridge in the carpool lane but now it's $2.50).
You can see the map of current pickup spots, get etiquette questions answered, see reports of problems, and more, at the casual carpool site: http://www.ridenow.org/carpool/
Some say that starting a new casual carpool pickup location is akin to starting a new LocalWiki.