Lots of museums offer 1 free day a month. Sure these days can get a little crowded, but it is free and some of these museums have a pretty steep entrance fee. Most often they are the first ____ of a month (for example, Asian Art Museum is free the first Sunday of every month)
Here is a listing:
First Sunday:
Asian Art Museum
First Monday: none
First Tuesday:
Cartoon Art Museum
655 Mission Street (between 2nd & 3rd)
First Tuesday is “Pay What You Wish Day.”
de Young Museum
Golden Gate Park
Also: $2 discount on regular admission with Muni pass or transfer.
Legion of Honor
Lincoln Park
Also: $2 discount on regular admissionwith Muni pass or transfer.
San Francisco Museum of Modern Art (SF MOMA)
151 Third Street (between Mission and Howard)
Also discounted every Thursday evening: Half price admission from 6:00p to 9:00p.
Yerba Buena Center Galleries
701 Mission Street
First Wednesday
Exploratorium
at the Palace of Fine Arts
Always Free:
Cable Car Museum
1201 Mason St
Chinese Cultural Center Gallery
750 Kearny St, 3rd Floor
Museo ItaloAmericano
Fort Mason Center, Building C
San Francisco Fire Department Museum
655 Presidio Ave (at Pine)
Visitor Center & Hyde Pier at San Francisco Maritime National Historical Park
499 Jefferson St (at Hyde)
Wells Fargo History Museum
420 Montgomery St (between California & Sacramento)