Buy a Muni 1 ($11), 3 ($18) or 7-day ($24) visitor passport for unlimited rides on cable cars, streetcars (trams), trolleybuses and diesel buses, but not BART. Regular fare is $2 (Cable Car is $5).
If you're 65 or older, show your driver's license, ID or passport as proof of age and buy a Muni monthly Senior Pass ($15).
Muni's visitor passport and Senior Pass can be purchased at their kiosk (looks like a cable car) at the Powell-Market cable car turntable. You can also buy a Muni street and transit map for $3 there.

Muni (San Francisco Municipal Railway)
Phone: 311 (w/in San Francisco)
(415) 701-2323 (outside San Francisco)
www.sfmuni.com
Visitor Passport
http://128.121.89.101/cms/mfares/passports.htm
Senior Pass
www.sfmta.com/cms/mfares/passes.htm

There's more standing room (that's half the fun!) and brilliant views on the less well-known California Line, running on California Street between the Ferry Building end of Market Street and Van Ness outside the Holiday Inn. It's always easier to get on than the Powell/Mason lines, and free with a MUNI passport.