Today (Thursday June 21 2006) was "Spare the Air Day" in the San Francisco Bay Area. Most transit in the Bay Area was free to anybody. Caltrain didn't seem that much busier than normal during the morning and evening commutes.
Friday is another Spare the Air day. My wife and kids will take advantage of the free public transportation by taking Caltrain into San Francisco, SF Muni to the Ferry Building, then the Ferry to Sausalito.