Card holders can earn points with their purchases that can be used toward free BART tickets or even cash. Using the card to buy BART tickets earns 5 points per dollar. Spending $500 will get you a free $25 BART ticket. There's no annual fee but the interest rates are pretty steep. If you live in the Bay Area, ride the system and pay off your card every month, you might want to consider one of these.
[Source: San Jose Mercury News, thanks to Damon for the tip]