You paid the meter but neglected to curb your wheels. You overslept and didn't move your car for the street sweepers. You're allergic to metal and can't carry the two pocketfuls of coins it takes to fill up a meter these days. Whatever the reason, if you drive in San Francisco, you inevitably get parking tickets, and fork over 35 or 50 bucks to the city with alarming regularity. But there's a little-known loophole for people who are short on cash but long on free time: Project 20, which lets people pay their citations through community service. Would-be participants must first go to the Department of Parking and Traffic at 1380 Howard St., where you sign a contract outlining the terms (the rate is $6 credit for every hour worked). Then head over to the Project 20 office to pick from a database of charities that are accepting volunteers.